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
Remove Nimble from framework search paths.
Use a single reference for Nimble.framework
CONTRIBUTING: Use .xcworkspace

- Advise contributors to use workspace file.
- Remove "BDD"--it's a loaded term. You can use Quick to write
  non-behavioral tests, just as you can use XCTest to write behavioral
  tests. I believe it's often used to denote "xSpec"-style frameworks,
  but I'd prefer something less ambiguous.
- Remove notes on adding files to both iOS and OS X targets. CI script
  now tests both iOS and OS X builds, so it's unlikely an error like
  this will make it into the build.