Move Nimble dependency to a workspace

This creates a workspace to hold both the Quick and Nimble projects. Nimble is
a dependency for the tests, but isn't needed in the main framework targets.

Having Nimble embedded in Quick.xcodeproj was causing issues with
[Carthage] building both Quick and Nimble. The end result of the issue was
that if you included both Quick and Nimble, only the version of Nimble
included with Quick would exist after compilation. By moving it into a
workspace, we can ensure that Carthage will only build the Quick framework,
letting the user manage Nimble separately.

[Carthage]: https://github.com/Carthage/Carthage

Related discussion on Carthage: Carthage/Carthage#108