Added PropType validator for componentClass to ensure the prop satisfies React.isValidClass

[react-bootstrap](https://github.com/react-bootstrap/react-bootstrap) is currently using a [custom](https://github.com/react-bootstrap/react-bootstrap/blob/master/src/utils/CustomPropTypes.js) prop validator that ensures the prop satisfies `React.isValidClass`. I am finding that I will need the same functionality in another project without a dependency on react-bootstrap. This would be a nice validator to have in core. I'm open to renaming it to whatever sounds better to you, just let me know.

@pieterv