Merge pull request #15 from tvarbanov/master

Multiple enemy,enemy object fix,enemy and shot collision
Added GameOver, some enemies changes
Merge pull request #17 from Enlightment/master

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

Enemies
Refactoring: collision checking for shots and enemies out of Factory classes and optimized
Merge pull request #19 from Enlightment/master

Refactoring: collision checking for shots and enemies out of Factory classes and optimized
Refactoring: better gameplay, some bugs fixed
Merge pull request #20 from Enlightment/master

Refactoring: better gameplay, some bugs fixed
Enemy minor fix