[Mobi] Mobi output improvements

1. Produce dual Mobi file by default (Mobi7 + KF8). Much nicer and
richer experience on Kindle devices.
2. Embed fonts in Mobi.
3. Remove obsolete Mobi stylesheet (mobi.py uses epub css by default)
4. Add flag with-full-fonts to epub.py as it is required for mobi.py to
produce working subset of fonts by calibre.
5. Ignore margins option by default to ensure that result mobi file
will use Kindle devices own margins.