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 just tried this out and it seems to be invoked once for each spec.

For code in `+initialize` that should only be executed once, [the documentation recommends](https://developer.apple.com/library/mac/documentation/cocoa/reference/foundation/classes/nsobject_class/reference/reference.html#//apple_ref/occ/clm/NSObject/initialize) testing if `self` is equal to the base class—so something like this:

```objc
+ (void)initialize {
  if (self == [QuickSpec class]) {
    [Word runBeforeSuite];
  }

  [self exampleGroups];

  if (self == [QuickSpec class]) {
    [Word runAfterSuite];
  }
}
```