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!
Thanks for the tip @sharplet. 

I'm a little bit confused as to how the `QuickSpec` class works (I actually have more experience with Swift than Obj-C). I have some more time later this afternoon to try to dig deeper.