Microarchiver

* Add missing Python package requirement to requirements.txt.
* Simplify PMC upload script.

* Fix broken logos and images in README.md.
* Replace local version of `debug.py` with the use of [Sidetrack](https://github.com/caltechlibrary/sidetrack).
* Use newer approach to recording version and other metadata in `__init__.py` and the release procedure codified in `Makefile`.
* Minor internal changes.

* Fix [issue 2](https://github.com/caltechlibrary/microarchiver/issues/2): volume number in file names is incorrectly determined

* Support output for PMC using new command-line option `-s`.
* Rename the JATS XML file after the pattern _issn_-_volume_-_doi_.xml, to make it more compatible with output generated for PMC.
* Remove any alpha channels and convert images to RGB.
* Run ZIP files through a simple validation step after creating them.
* Added missing dependencies to [`requirements.txt`](requirements.txt).
* Some internal code changes.

* Instead of quitting with an error if the file given to `-a` is empty, `microarchiver` will now just print a warning.

* Store JATS XML for each article, as well as any image referenced in the JATS data. Images are converted to uncompressed TIFF before being stored.
* Perform JATS validation for each article by default.
* Add `-X` option to disable automatic JATS DTD validation.
* Change `-a` option to accept a file containing either a list of DOIs or the XML format sent by micropublication.org.
* Change exit code numbering scheme.
* Communicate number of failures in terms of exit codes; see [README](README.md) for more details.
* Test if date given to `-d` is syntactically correct but not a valid date.
* Refactor and change some internal code.
* Fix miscellaneous bugs.