Update plugin to maintained version and add simple_form input

Hi, Matteo!
I do some changes to your gem.

1. I've added to your gem simple_form input, which is loaded only when needed.
2. But after that, I've found out that your gem is really outdated. And changed colorpicker plugin to maintained one, which works better and have better effects.
3. Also I've changed directory for assets from app to vendor again. It should be vendor, because plugin is not yours. Cause of that I've added images precompilation to your engine.
4. And finally, I've changed readme accordingly.

Please consider these changes, and if you like it please release new version of the gem.

Thanks you for your gem.
Pavel