Append and modify CSS when sending epubs to Kobo devices

This allows users to have a "kobo_extra.css" file in the root of their
device containing CSS rules. This will be appended to all stylesheets in
the epub. As well, if the extra rules contain an @page rule, any
existing @page rules will be stripped from the stylesheets. Finally, if
any of the extra rules include "widows" and "orphans" settings, these
are stripped from the rules in the stylesheets.
Kobo driver CSS modification should use mimetype
pep8 cleanup of kobo/driver.py

Kobo driver: Add an option to modify the styling in books being sent to
the device, based on a template on the device. Fixes #1207151 [Append and modify CSS when sending epubs to Kobo devices](https://bugs.launchpad.net/calibre/+bug/1207151)