Fixes and Sprites

BaseSprite can be used for locations, but doesn't have any of the rendering hooked up.

Fixes:

• Scenes needed to return False to continue operating. Now need to return True which is more intuitive. https://github.com/pathunstrom/pursuedpybear/issues/45
• Return None for next scene would crash the engine. Now can return None when you don't want to replace the current scene. https://github.com/pathunstrom/pursuedpybear/issues/44
• Flickering in Windows platforms resolved. https://github.com/pathunstrom/pursuedpybear/pull/58
• No longer need to set Sprite.dirty to True each frame.

Version 0.2

A complete redesign of the core engine.