Adds an HTML 5 color picker form field type

This PR adds an HTML 5 color picker form field.

This includes the necessary changes as per the spec here:

http://www.w3.org/TR/html5/forms.html#color-state-(type=color)

Includes unit test.

Add this to a form using:

<field name="color" type="color"/>

Cheers,
Eric.

PS: Not sure if I have done this PR correctly.... still trying to get used to using Github for this sort of stuff.