Cleanup / Update Bodhi module
Add a few comments, cleanup errata module
Small fix to gem cache location
Flush out Polisher::Gem#diff, add specs for Gem module

Diff will unpack local and specified gems, do recursive
diff against contents and return result.

Next step would be to pipe into a formatter object to
format rpm spec patches or anything else.

Functionality still needs to be flushed out at Gemfile
level, eg recursive diff for vendored deps & such

Specs have been written for Gemfile module, though
some are stubbed out as to be implemented