Allow building branches other than master

[ferrouswheel]

Should be able to build any branch that gets updated, as well as actually filter on branch name.

Unfortunately to do this write would require evaluating the UI too:

It'd be unclear which branch is failing if one just shows all builds in a chronological list. I'd prefer to have a separate history for each branch.

Also need to consider:

• How to show the status of building pull requests etc.
• What to show on the project overview page (just the master branch or the one selected in the project config?)