Move Class Creation Concerns into a ReactClass Module

This is in preparation for the separation of Composite Component internals from the Class abstraction.

This moves logic responsible for class creation and mixins into the ReactClass module. This currently creates a class that extends the ReactCompositeBase. In the future, this will be decoupled as ReactCompositeComponent will compose these classes.

This is just a cut/paste and renamed some "ReactCompositeComponent" sites to "ReactClass".
I'm leaving these tests in this file. I want to make a separate pass of moving unit tests into the right location once things stablize. Some will be replicated for ES6 classes etc.