[WIP] Integrate Arbiter

[jspahrsummers]

Resolves ArbiterLib/Arbiter#62.

To do:

• Documentation
• Fix Arbiter's build order so that ReactiveTask is no longer building before ReactiveCocoa
• Refactor Arbiter's support for commit pinning, since it doesn't work for Carthage
• Restore Git reference support
• Restore support for lastResolved and dependenciesToUpdate
• Improve error reporting
• Pin to Arbiter master

[jspahrsummers]

Made some good progress here, but there are some memory errors, and not all Carthage features are supported yet.

(And, of course, Arbiter only works serially right now, so I expect performance/UX to be worse.)

[jspahrsummers]

Was just now able to run this on a fresh clone of Carthage. 🎉

It mostly worked. The commit pin for Nimble was ignored (as expected, because I didn't implement it yet), and the build order was slightly wrong, but version selections were otherwise correct.

[jspahrsummers]

Build order is fixed. 👍