Make it easy to checkout GitHub pull requests

This setting makes the following workflow possible:

Look at changes:

    git co-pr 123
    git diff origin/master

Bundle, validate tests are passing:

    bundle
    rake

Rebase interactively, squash, and potentially improve commit messages:

    git rebase -i master

Merge code into master:

    git checkout master
    git merge pr/123 --ff-only

Push:

    git push origin master

Clean up:

    git branch -D pr/123