Merge pull request #17 from Enlightment/master

Added GameOver, some enemies changes
Merge pull request #18 from Enlightment/master

Enemies
Merge pull request #19 from Enlightment/master

Refactoring: collision checking for shots and enemies out of Factory classes and optimized
Merge pull request #20 from Enlightment/master

Refactoring: better gameplay, some bugs fixed
Enemy minor fix
Merge pull request #21 from tvarbanov/master

Enemy minor fix - Enemies Layer Redrawn after enemy removed