Introduce React Class as an alias for Composite Component

This is in preparation for the separation of Composite Component internals
from the Class abstraction.
Move Class Creation Concerns to ReactClass

This moves logic responsible for class creation and mixins into
the ReactClass module. This currently creates a class that extends
the ReactCompositeBase but in the future, this will be decoupled as
ReactCompositeComponent will compose these classes.

This is just a cut/paste.
Merge pull request #2396 from sebmarkbage/classmodule

Move Class Creation Concerns into a ReactClass Module