fix infinite loop caused by duck-punching coffee-script more than once

Because of the way that `coffee-react` is duck-punching / monkey-patching the `coffee-script` module, it causes an infinite loop if two distinct of `coffee-react` modules are `require()`'d and then you attempt to parse compile `.cjsx` files into JavaScript.

This took a long time to debug but the fix was pretty simple. All I did was add a `_cjsx` property to the `CoffeeScript` object. If it already exists, it won't try to modify it a second time. I would be very happy if this could get merged in, and `coffee-reactify` could be updated to use the new version.