Refactor Srcset Parser
Refactor Srcset Parser

Now passing jshint and jscs, and moved utility and reg ex definitions into higher scope for reusability. Unfortunately the comments from the spec are now all out of order, so that functions aren't called before they're defined. Kind of sad for readability :/ but probably the right thing to do. @afarkas let's look at handling the "set" assignment outside of the parsing function proper..

scottjehl/picturefill,scottjehl/picturefill