Introduce React Class as an alias for Composite Component

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.
This now does `ReactCompositeComponent.Base.hasOwnProperty(name)` which has some more names of internals on it than the other one. This is a bit more restrictive but hopefully nobody relied on being able to override internal names that wasn't on this particular mixin.