Groups implemented

Implemented groups as requested in #12.

Groups are defined in the reviewers string with semicolons:
``` ruby
ENV["REVIEWERS"]  = "johannes@hyper.no:jgorset@gmail.com;ruby;javascript,tim@hyper.no,harald@ringvold.nu;javascript;ruby"
```
This could probably be defined some other way but I tried to follow the existing pattern.

The groups for a repository are defined in the post-recive hook after the service paramater:
`/?service=github&groups=ruby,javascript`

If the repository does not define a group only reviewers with no groups are eligible to review the commits from that repo.