Lots of stuff

-Projectiles now fully implemented (GTDProjectile, GTDUnit, GTDMap) look
at code for details.
-Projectiles and units now use SDL_image, adding support for every
popular image format (previously only BMPs allowed).
-Altered tooltips for speed assist and dmg assist towers since they
don't attack. (GTDGame)
-Waveunits now rotate properly depending on which direction they are
facing. (GTDMap)
-Fixed issue causing attack radius circles to be drawn for waveunits
(GTDMap)
-Completed static functions for different unit properties (GTDUnit)
-Other minor changes here and there