carthage 0.3 (new formula)

Adds a formula for v0.3 of carthage.

cf https://github.com/Carthage/Carthage/issues/19
Generally the `test do` we use for ensuring that e.g. everything was installed and linked correctly. Ideally you'd e.g. install a package into a local directory (`test do` creates and changes to a temporary directory and everything created locally is deleted after running).