Renamed styles attributes:

 progressText -> textProgress
 completeText -> textComplete
Added new attribute to control button corner roundness:

cornerRadius
FlatButton class code clean up.
Code clean up.
Re-factored ProcessButton:
- now state depends on progress:
- - normal state [0]
- - progress state [1-99]
- - success state [100]
- added abstract `drawProgress` method to simplify creating child buttons
Added error state to ProcessButton:
- error state [-1]
Added button state sample