Add an option that lets you specify the "master" branch.

If you didn't have a branch named "master" `git cleanup` would die and throw a stack trace. Now it'll show an error, and allow you to specify a different name for your master branch.
Show an error instead of a stack trace when running in a directory without a git repo
Merge pull request #8 from praxxis/master

Add an option that lets you specify the "master" branch