Libgutenberg

- The `heading` column in the database's author-book many to many table was being ignored by much of our code. The result was that multiple authors were being listed in alphabetical order. now, the heading column is used and the first sort column for the authors of a book, and the authors other than the first author are have heading=2 (instead of the default `heading=1`) set on initial metadata load. The cataloguer can reset the heading numbers, but does not wish the order of authors other than the "main" author to be tracked in the database.

- fixed a reversion in 0.10.10 that made author name matching case sensitive.

- credits should be replaced, not appended
- added test for credit replacement
- fixed tests for current db
- 0.10.21 was not deployed

- add credit cleaning to GutenbergDatabaseDublinCore. required for autocat3 https://github.com/gutenbergtools/autocat3/issues/116
- refactor credit cleaning
- refine pretty_title truncation to always add … to truncated titles. improves 33
- change no-gutenberg-id log message from warning to info, improve wording fixes 35

- add media type for `.zip`
- add aliases to metadata parser for `author of introduction` and `author of afterward'.
- fix off by one error in rendering of BCE dates: 1 BCE is represented as 0.
- fix metadata parser not to raise an error only because there's no ebook number - a title suffices.
- ensures no markup in 'language attributes. this was crashing ebookmaker.
- tests added for DCMIType
- improves title truncation in dc.make_pretty_title(). fixes 33

- add handler for 'original publication' in pg header. This field is not parsed.