Provide a way to configure QB to only process pull-requests that are destined for a specific branch.

When user submits a pull-request, they request it to be merged to a specific destination branch (which is indicated at top of pull-request page on GitHub; e.g. https://github.com/01org/libva-utils/pull/44). We want to be able to have QB only process pull-requests for, say, "master" branch and not feature branches.