Added Enemy Interface and Factory. And some tests using gtest.

Hey @walsh06 @SheepishMonster , 
Added an Enemy Interface, and a Robot Enemy who implements this interface.
The EnemyFactory then instantiates or "creates" Robot Enemies and can create enemies with varying health and power. 
I used gtest as our Test Framework. Pretty easy to use after you get it set up. But this can still be discussed.

**Questions**
Instead of having the EnemyInterface hold health and power should we have a "EntityInterface"? This way the Player could also implement this. I guess we need to decide what our player does and what our game is. For now, this should do.

**TO DO**
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