Add support for zip archives.

Additionally, this adds support for bz2 compression and uncompressed tar
files.

Any unknown format falls back to tar.gz.
Update Gemfile.lock

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge pull request #20 from jhollingsworth/feature/zip-archive

Add support for zip archives.
Fix comment to Gitlab::Git::Repository.new in README
Allow to specify diff paths