Reduce starting speed and acceleration
Increase chance and variety of obstacles
New tile graphics
Remove unused colour constants
Update colour palette
Change font to Lato
Add more fine grained mouse events.

New events are mouse_click, mouse_drag_start, mouse_drag_move and
mouse_drag_end. This allows objects to respond differently to clicks
and drags which we'll need for rotating tiles.
Use the new mouse drag events in Tile
Rotate Tiles in the Rack when clicked
Update version to 0.2.0