Implement a createClass method.

Rationale: Components made with typed-react should be usable by JSX
programs using JSX syntax. JSX, starting in React 0.12, transforms
`<!Component />` into `React.createElement(Component, null)`.
React.createElement does not create a factory, but a component. For
more information, see https://gist.github.com/sebmarkbage/d7bce729f38730399d28#comment-1309190 .

You can read more about the new React DOM Terminology at
https://gist.github.com/sebmarkbage/fcb1b6ab493b0c77d589 .

Potential confusion could be caused by the fact that with this change
typed-react's createClass and createFactory both return
ReactComponentFactory<P>, although the result of createClass is not a
function. I can't think of a way around this given that they
quack the same way.

This change also adds React 0.12's createElement function to react.d.ts.
This makes the test case more clear and allows for developers to choose
whether to use factories or React.createElement explicitly. A pull
request will also be sent to borisyankov/DefinitelyTyped, but it would
be good to get your feedback first.

Test plan: Added a test for the new createClass function. Verify that
the resulting component works.