Add the ability to get info about a single branch.

I thought about adding Pithub::Repos::Branches.  I decided against it.
* The API docs don't break it up that way.
* There's only two things you can do with branches, list them and get one.
* Pithub::Repos::Branches can always be added later and these be made into
  compatibility wrappers.

For #171
Merge pull request #172 from plu/feature/branch

Add the ability to get info about a single branch.