RailsBricks 3 proposed changes, with MySQL support

Hi Nico,

As I've told by email before I'm sending you this pull request so you can check and review the changes and get into your repository if you like them (I hope so :)). Also, I've done my best to avoid modifying any existing features, so everything should work as is, only with more features now.

Please let me know about any required changes and/or opinions, I'd love to hear them from you.

p.s.: also, you should note that I've put "you will need RailsBricks version 3.1.0 or later to use it with MySQL" in my guide assuming that this was the next version number you might use, so please make sure you modify RailsBricks version numbers accordingly if you accept the changes (or let me know and I'll modify the guide) so it keeps in sync with the guide.

Best Regards,
David Camarena