Use IntDef for float label modes to have lint and better IDE autocomplete support

One more pull request :)

Since the library uses android's annotations support library, you can take advantage of `IntDef` to better lint floating label modes. Users' IDEs will recognize this annotation and offer to autocomplete with them and also show an error lint if they don't pass in one of the predefined modes. Similar to how `Toast` requires `Toast.LENGTH_SHORT` or `Toast.LENGTH_LONG` even though they're just ints.

![screenshot 2014-11-26 00 46 54](https://cloud.githubusercontent.com/assets/1361086/5198465/334cb6a4-7506-11e4-8e38-b8d50b8da2db.png)