New simplified design

[bvssvni]

• Draw coordinates should be computed manually using window back-end
• Separating state from button
• One enum for buttons
• Another enum for motion, which can be easily extended to gamepads or tracking devices
• Added text input

Closes https://github.com/PistonDevelopers/input/issues/35 Closes https://github.com/PistonDevelopers/input/issues/34

[bvssvni]

@tomaka I am going to merge this since it solves some urgent problems in Piston and it is blocking Conrod.