Compile (the original) RBush with ol

This PR removes the custom (re-)implementation of the RBush and instead integrates the original RBush library into the build. This makes it easier to follow improvements.

Basically it works like this:
 - A dependency is added to the rbush node module.
 - We copy the file `node_modules/rbush/rbush.js` to `build/ol.ext/rbush.js`.
 - Then we add the line `goog.provide('ol.ext.RBush');` so that it can be `goog.require`d from the ol code.
 - To make it compilable, we also add a magic `@suppress { ...  }` to ignore all [compiler errors](https://code.google.com/p/closure-compiler/wiki/Warnings).
 - That's it. Now the original RBush library will be included in the build (also minified for `ol.js`).

Comments welcome!