Add Objective-C Support

This has been oft-requested (see #1980 and the [wishlist](https://github.com/codemirror/CodeMirror/wiki/Mode-wish-list)).

![image](https://cloud.githubusercontent.com/assets/381393/4966800/9bbc49d2-67e3-11e4-9c6d-b435266fcf41.png)