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!
Like `beforeEach` and `afterEach` blocks, I'd like users to be able to specify an arbitrary number. That is, the following, split among three files:

```swift
beforeSuite { println("One!") } // in SpecOne.swift
beforeSuite { println("Two!") } // in SpecTwo.swift
beforeSuite { println("Three!") } // in SpecThree.swift
```

Should produce the following output prior to any examples being run:

```swift
One!
Three! // We make no guarantees as to the order
Two!   // in which the closures are evaluated.
```

This kind of behavior is taken from RSpec.