Enemies are now Sprites too.

Three down and one to go.  GrassBlock, Player, Enemy are now all Sprites.  Clouds will be harder.

Then the Scene logic will become MUCH simpler.

New bug: Enemies no longer kill at all!  But this will be rapidly fixed.
Merge pull request #18 from cclauss/patch-5

Enemies are now Sprites too. Closes #3 and #11.