First checkin of Singleton Code for Manager classes.

Includes Arena and Manager classes, both singleton
manager-types. Roles of each to be determined.
Added wave logic to the Arena.

Waves come in after a designated amount of time. Will
need to improve on the spawning logic later.
Fixed HUD scripts attachment to HUD objects. Prefabs updated.
Textures folder deleted, including meta file.
Merge branch 'master' of github.com:redbluestudios/gladiators