carthage 0.3 (new formula)

Adds a formula for v0.3 of carthage.

cf https://github.com/Carthage/Carthage/issues/19
From the looks of your README the ideal would be:
1. create a `Cartfile`
2. run `carthage` to download and install a single package from there

Assuming you use normal return codes that'll be enough; `system` will error-out if there's a bad return code.