Clean way to multithread with pygame. (Fix #4 #5)

Remove custom threads and only render from within the on_execute loop.
According to the following posts, no custom thread should ever render a GUI
element on its own:

+ https://groups.google.com/forum/#!topic/wxpython-users/_l66b-woPM8
+ https://groups.google.com/forum/#!topic/wxpython-users/0hnaFna0HZ0
+ http://stackoverflow.com/questions/7141665/multithreading-with-pygame