Add color_field wrapper

* new casein_color_field method which wraps new rails color_field helper

Looks like my editor may have ripped out a bunch of tbas leading to what looks like a larger change than it really is.  The key is on line [196](https://github.com/rcode5/casein/compare/add-color-field-helper?expand=1#diff-fb4605bee8a4c6cb2a6ede29cffd255bR196)