carthage 0.3 (new formula)

Adds a formula for v0.3 of carthage.

cf https://github.com/Carthage/Carthage/issues/19
This was intended before Xcode 6.1.1 or Xcode 6.2 was released. I'm not sure what the best path is here, if you can assume all users are forced to be on the newest version of Xcode this could be removed. Carthage did not build in 6.1