Fixes #509: pass callback on setState and setProps

[alecrobins]

Resolves enzymejs#509

Due to the asynchronous nature of setState, we can't assume a wrapper.setState or wrapper.setProps (which uses setState in the setChildProps call) will act synchronously.

This PR adds callback functions to the stateState and setProps functions for full DOM rendering and setState for shallow rendering.

Example:

class Foo extends React.Component {
  constructor(props) {
    super(props);
    this.state = { id: 'foo' };
  }
  render() {
    return (
      <div className={this.state.id} />
    );
  }
}
const wrapper = mount(<Foo />);
expect(wrapper.state()).to.eql({ id: 'foo' });
wrapper.setState({ id: 'bar' }, () => {
  expect(wrapper.state()).to.eql({ id: 'bar' });
});