Added gravity to button menu

Sets a gravity for children of the floating action menu.
Children can gravitate to start, center or end of their corresponding x or y position, depending on the expand direction. The text labels automatically get changed position from left to right based on the gravity setting.

How it looks for start and end: 
![screenshot_2014-12-24-21-16-13](https://cloud.githubusercontent.com/assets/3596362/5550970/2da41136-8bb6-11e4-8a7d-ee92d91aae55.png)