Add support for :as => :color to input fields.

This is straightforward for Rails 4, but for Rails 3 we have to patch
a color_field helper in to ActionView as it was only introduced in Rails 4.
Raise an error when used in Rails 3.

Rather than patching a color_field method in to ActionView.
Merge pull request #1039 from pipt/color-input

Add support for :as => :color to input fields.