Adds IE background color fallbacks

Adds fallback solid colors for gradients for IE.
Fixes bug form submission bug on custom value

When form was submitted the script was looking for the add input on
fieldsets that didn't have an add input field.
IE polyfill adjustment

Simplifies IE8 checkbox polyfill to fall back to regular (unstyled)
radio buttons because IE8< doesn't support :checked selector.
Updates tests and cassettes