Issue #39 - Activate Raw Product Description if it is present.

This pull request is dependent on [another pull request in Spree Core][1] which I am hoping gets accepted.

This change fixes Issue #39 but it is implemented in a way that the code still remains backwards compatible with older versions of Spree. It can be back-ported to any version that you want as a result. It also means that you can merge this in right now and it will not cause any problems. I would recommend that it is brought into the branches 1-3-stable and master. 

I would also suggest that you update the README file and let people know that, in order for the CK Editor to work correctly, they need to a version of spree that contains this preference fix.

This change contains no tests, sorry about that one. I hope that you like it though and please let me know if I can fix anything. Cheers and thanks for spree_editor! It rocks, I love it.

 [1]: https://github.com/spree/spree/pull/2874