Xcode 6 Beta 5 compatibility.

Straightforward changes include:
- `@auto_closure` to `@autoclosure`
- `Sequence` to `SequenceType` and `GeneratorType` to `Generator`
- explicit nil checking (optionals no longer conform to `BooleanType`)
- because of the above change, removed optional truthy/falsy tests

Less straightforward (controversial?) changes...
- Removal of `Equatable` on `Array`... I couldn't get this working in a straightforward way because `bridgeToObjectiveC` was removed (which is how equality was being checked before). It isn't currently possibly to add a generic constraint either and it also isn't possible to downcast `Array<T>` to `NSArray`. I ended up changing the tests to use `NSArray` for equality checks for now.

I'd definitely welcome thoughts regarding Array and Equatable... I realize that isn't the most ideal change, but I also didn't think it made a lot of sense to add an expect implementation whose sole purpose was to handle `Array<T: Equatable>` either. Maybe that would be best, though.

Thoughts?