Flat button style

Added a new button style named "flat", which hides the button border and background. This allows several shortcut controls to be placed into a table or other bordered view with a minimum of visual clutter.

![screen shot 2014-11-24 at 8 09 44 am](https://cloud.githubusercontent.com/assets/249247/5165353/5b4c3d24-73b1-11e4-8508-d576631887d6.png)