+ Made sprite loading more robust.
  * src/explosions.c:
    + No longer use ee_putchar etc. to draw our sprites, but use
      ee_draw_sprite().
  * data/:
    + Drew explosion_medium and explosion_small sprites.

+ More robust sprite loader.
    + Added ee_set_sprite_frame() and ee_get_sprite_frame().
    + Free all structures in ee_free_sprite().
  * src/aliens.c src/bonus.c:
    + Use ee_draw_sprite() instead of our manual sprite rendering.
  * src/box.c:
    + Use ee_draw_line() instead of the manual equivalent.
  * data/:
    + Added foo_fighter, baz_fighter, item_gem and item_heart sprites.

+ Wrote a little sprite demo.
  * data/:
    + Added bar_fighter (from ttyvaders) and bar_boss (from shapes.txt).