Add before and after suite

Addresses issue https://github.com/modocache/Quick/issues/2. 

Adds optional closures that can be run before and after the suite.

Comments are inline. I'd love to get feedback!
> I actually have more experience with Swift than Obj-C

Wow, that's awesome! Welcome to Swift and Objective-C! :metal:

`QuickSpec` is forced to do some complicated stuff in order to play well with XCTest. It's kind of intimidating at first glance, so I added some documentation in 5570d548c17864f4e818a440519aa22ae25e3aa0. Hope that helps!