[WIP] Use Carthage for submodule management

_Depends on https://github.com/Carthage/Carthage/pull/118._

This allows RAC to be used nicely as a Carthage dependency, without adversely affecting anyone who doesn't use Carthage.

**To do:**

 - [ ] Use `xcconfigs` outside of Carthage?

/cc @mdiep @robrix @alanjrogers 
Oh, target of opportunity? Ok.