Rename asteroids to asteroids.py

My game utilizes the simplegui module developed by Scott Rixner, professor of Computer Science at Rice University.  To most easily launch my Asteroids game, utilize his CodeSkulptor IDE, which implements the simplegui module.

Asteroids game: http://www.codeskulptor.org/#user16_ZCN3sDT4sU_14.py

Note: press the play button (right-facing triangle) at the top left of the CodeSkulptor window to execute the code.