Added RegExp support

Hey there, bothering you again with a Pull Request. Hope you won't mind.

I added RegExp support for the `where` queries. Thought it might be useful. As with the previous one, I provided a test, all your previous tests pass, and updated the documentation.

If you don't think it should be included, no problem, it's just that I need it for the sails.js adapter (which by default makes case insensitive queries) so I thought you might want to benefit from it as well.