Add Gravity support for floating label

Now you can use left, right, center gravity for floating label. When using RTL languages such as Persian, Arabic, Hebrew and etc. gravity option might be useful.


![screenshot_2014-02-24-13-24-37](https://f.cloud.github.com/assets/1250691/2244156/060521ee-9d3a-11e3-91c7-e6b90582635d.png)