DSL: Support waitUntil in Objective-C

Support waitUntil in Objective-C.

- Split waitUntil DSL into its own Swift file.
- Convert original waitUntil functions into class methods in order to
  bridge them to Objective-C using the `@objc` keyword.
- Define Objective-C DSLs to call the newly created class methods.
- Add objc tests for the new DSL.
- Update README.

Fixes #71.
I think this could probably just use a boolean to indicate whether to do the `dispatch_async`. Unless the block passed to `pollBlock` can be called concurrently, which doesn't look to be the case.