carthage 0.3 (new formula)

Adds a formula for v0.3 of carthage.

cf https://github.com/Carthage/Carthage/issues/19
Yep, exactly. Similarly with `make package` above; take a a variable and know to Do The Right Thing rather than having to spell it out as verbosely. Ideally things look like:
```ruby
system "make"
system "make", "install", "PREFIX=#{prefix}"
```