Issue 3888: Mobi/epub downloads now use calibre

THIS REQUIRES CALIBRE TO BE INSTALLED IN /opt/calibre (the default install location) OR IT WILL NOT WORK

https://code.google.com/p/otwarchive/issues/detail?can=2&q=3888

Switch to using calibre for mobi/epub generation. 
Refactor downloadable functionality out of Work into Downloadable module
Switched from Iconv to ActiveSupport::Inflector.transliterate(string) for converting to ASCII for filenames
Added/edited lots of tests. 
Cleaned up deprecated calls.