Implemented Health, Time Based Waves, and the DFS

Health script is attached to each enemy object, it decreases each time
it is hit by an "ammo" tagged object. Waves run at set intervals, but a
new wave will start only when all the enemies have been destroyed at a
given wave. The DFS pathing seems a little weird since it looks like it
takes its time looking for the target position.
Merge branch 'master' of https://github.com/mrome007/cs179ProjectVer1

Conflicts:
	Assets/Prefabs/TowerBasicAmmo.prefab
	Assets/Scripts/Buy_Shoot_Modes.cs
	Assets/Scripts/DestroyOnContact.cs