DSL: Support waitUntilTimeout in Objective-C

Support waitUntil with timeouts 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.

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I tried defining `NMB_waitUntil` as well, but for some reason it just wouldn't work. The compiler kept complaining that the block passed to `NMB_waitUntil` was an unused parameter. I couldn't make heads or tails of it, so I decided to issue this PR for just `NMB_waitUntilTimeout`.
Wow, I must have been extremely tired when I wrote this. Thanks for pointing this out! It turns out this was the cause of my troubles with `waitUntil`. :+1: