allow string values like 'sans-serif'

and 'border-box'
also, print the values when there is a TypeError as this is much more helpful
support CSS3 attribute exact equality selector

http://www.w3.org/TR/selectors/#attribute-selectors
input[type="text"]
support rgba(...) values
handle selectors_group

.class1, .class2 { ... }
use defaultdict for selectors
Merge pull request #1 from raylu/master

Merge