Delete the PR branch after merging

[contolini]

Via @Scotchester. See http://mikedeboer.github.io/node-github/#api-gitdata-deleteReference. The ref would be /heads/<branch-name>.

Before deleting the branch we should check that the base fork and head fork are the same AND that the head branch is disposable. That is, if someone is merging canary into master, they probably don't want to delete canary afterward if it's their permanent dev branch. If they're smart they'd make canary protected in GH so that it can't be deleted but we shouldn't rely on that. Perhaps we can view the repo's history to see if the branch has been used in the past and if it hasn't it means it's safe to delete?