Ebookmaker

- fixed missing subtype in link rel setter

- fixed cover setting issues in 0.11.15
- parser now converts unicode to Normal Form C. So "A"+"combining-`" -> "À"
- CSS serializer now omits invalid CSS properties in the derived HTML5. CSS profile used in CSS2, and includes a small number of properties that are marked as errors by the HTML5 validator because it considers them deprecated by CSS3 (for example, the "speak" property, which is replaced by "speak-as") We'll need to move to CSS3 eventually, but for now we need to also target EPUB2 and ereaders that don't do CSS3 yet. The supported CSS Properties are defined by profiles in the cssutils module in python; CSS3 properties are somewhat modular, and there needs to be discussion around which properties we should be using in PG files.

Not deployed due to test failures
- fix syntax and position of style element replacing HTML5 deprecated elements
- remove xml:space from HTML5 pre and style elements
- make removal of http-equiv meta elements case-insensitive
- try to remove problematic carriage returns in meta tags
- tralign, trvalign, tbodyalign, tbodyvalign changed to css equivalents
- remove imglongdesc; it never did anything
- add title to files produced from txt
- to allow for better HTML5 validation the preferred mechanism for denoting a cover image has been changed from `<link rel="coverpage" type href="a_relative_url.jpg" />` to `<link rel="icon" href="a_relative_url.jpg" type="image/x-cover" />`. type is optional unless there is more than one linkrel=icon. The issue is that "coverpage" is not an HTML5-registered link relation. The registered "icon" relation is described as "Imports an icon to represent the document." The "coverpage" mechanism will continue to be supported and does not need to be changed, especially for XHMTL source files.
- Ebookmaker now looks for unlinked cover files if there are none linked. Cover file names must contain the string "cover" and must an extension in '.jpg', '.jpeg', '.png', or '.gif'. Cover files must be in the same directory as the source file or one of its subdirectories. At some time in the past, cover files for display on the website were similarly identified by name. Some covers in the backfile were replaced by generated files when Ebookmaker added the capability of generating cover files. With Ebookmaker now identifying cover files, many of the unlinked covers should be restored. When a cover file is supplied, it is still a best practice to use a link element in the html file.
- adds utility functions CommonCode.dir_from_url and CommonCode.find_candidates to refactor directory walking and url to path conversions

- fix error when a style element is empty
- fix error when style contains non-ascii text
- fix issue with aux files not being copied to html destination
- use secure version of Pillow

- fix reversion in 0.11.10 leaving out added meta elements
- remove encoding meta element originating in HTML5 source from EPUB2 files, as it was causing validation failures.
- remove media handheld rules for HTML
- move content of table summary attribute to data-summary attribute
- width attribute on table and col is converted to css in a style attribute
- non-integer width or height on img is converted to css in a style attribute
- fixed epub builds for books with images in external css sheets
- the 'big' element is obsolete; changed to <span class="xhtml_big">. css is altered or added as appropriate
- a number of table attributes are obsolete in html5 and changed to the corresponding css styles: tablewidth, colwidth, tablecellpadding, tablecellspacing, tableborder, tdalign, thalign, tdvalign, thvalign
- html5 doesn't allow elision of dd in dl. Where they are missing, we add empty dd
- html5 doesn't permit carriage returns. these are replace by newlines when represented as numeric entities.
- libgutenberg dependency updated to 0.8.11
- fix issue of polymorphism in dc.languages. Without a db, it's a list of structs; with a db, its a related collection.
- ebookmaker will no longer ignore xml:lang or DC meta attributes
- fix windows path comparison - ebookmaker will behave properly when input file is in outputdir