Added - Difficulty levels. Beginner, Intermediate and Advanced.
Added - Bonus Reserve. Can now store multiple catches of continuous fire and double, triple and super weapons.
Added - New alien that, when promoted, can fire directly at you. These have been added to both normal levels (missed bonus life) and EOLs (already promoted!!)
Added - Normal sprites can now fired back (missed bonus bomb.)
EOL large invader now fires with greater frequency.
EOL now uses different sprite when firing (same as super weapon.)
Reduced circling invaders at EOL from 15 to 12.
General tidy up of some code. This should help speed on slower processors.