Update cssPropertyMatcher

[peteschaffner]

According to the spec, the ordering of individual background-* property values is arbitrary, so shouldn't the cssPropertyMatcher regex be a little more flexible?

I ran into this issue when declaring background-color before background-image in a background shorthand property. Here is a quick attempt to fix. The new tests are on lines 16 and 17.

The solution presented there is probably a little too loose, but take a look and see what you think.

Thanks!