prove issues parsing srcset URL descriptors pairs
use the spec's algorithm to process image candidates

the old parser didn't conform to the spec:
http://www.whatwg.org/specs/web-apps/current-work/multipage/embedded-content-1.html#processing-the-image-candidates
this fixes #8
Merge pull request #10 from tim-evans/parsing-algorithm

Rewrote parsing algorithm to conform to the srcset spec