Add support for pointer events

http://www.w3.org/TR/pointerevents/, only IE supports it at the moment it seems, you should be able to polyfill with http://handjs.codeplex.com/

Implementation is not finished:

1. I have reused EnterLeaveEventPlugin to power both Mouse and Pointer, feedback on my rewrite appreciated.
2. EventPluginUtils/ResponderEventPlugin, it seemed like I was supposed to add pointer to isStartish/isMoveish/isEndish, correct?
3. Haven't tested it beyond the very basics yet, but it works.

```
   raw     gz Compared to master @ b75d11c6039e4d2ab796bce2a1934e9e69b2b7e7
 +5142   +640 build/react-with-addons.js
 +2233   +449 build/react-with-addons.min.js
 +5148   +609 build/react.js
 +2235   +460 build/react.min.js
```