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!
Won't this be run once per spec? Each one of them will be initialized, after all. Try putting a `println` in one of the `beforeSuite` closures, it should only be printed out once.