Ver 2.0.0: Rename then/catch to then/success/failure.

This pull request is a breaking change from ver 1.0.0 (361fba6)
which will improve Swift's type-inference by adding `success()` method
to reduce too many overrides on `then()` method.

### Ver 2.0.0 Changelog (2014/11/18)

- `task.progress()`'s `progressClosure` type changed from `Progress -> Void` to `(oldProgress: Progress?, newProgress: Progress) -> Void`
- `task.then(fulfilledClosure)` is renamed to `task.success()`
- `task.catch(catchClosure)` is renamed to `task.failure()`
- `task.then()` is no longer used for fulfilled-only handling