Removes requirement of RigidBody2D #78

[riktothepast]

Removes requirement of RigidBody2D as is no longer being used by the character controller.

[prime31]

This is an interesting one. The Rigidbody2D most definitely doesn't need to be required anymore and certainly not stored in an instance variable. That being said, it is very important for getting trigger callbacks. Either the triggers or the CharacterController2D need to have one. I am not sure what the best way to handle that is. Maybe documenting it in the README so that folks that don't know Unity well understand they need to have a Rigidbody2D on either the CC2D or their other triggers.