Add a simple iframe test page
Convert data-url.html to a reftest
Eliminate string match allocation in url.rs
Provide an error message for URL parse failure
Accept data: URLs on the command line without URL encoding

This facilitates quick testing, e.g.

    ./servo 'data:,<div style="color: red;">hi</div>'
auto merge of #1672 : kmcallister/servo/tests, r=pcwalton

I'd like to have an `iframe` reftest as well but `iframe`s are currently broken.  Will look into that next.