Make it possible to specify branches as dependencies.

The tip of the branch is pulled, like the tip of the master branch is pulled if you don't specify a branch.

This makes it easier to develop modules that are just being converted to yotta, as you can have the yotta-ified version in its own branch and depend on it from elsewhere without having to tag releases.

Of course, you should be tagging releases anyway as soon as things are working.

(this fixes #85)