Ensure URLs returned are properly encoded

[aaronpk]

The change in #170 means it's possible to return URLs like http://example.com/foo bar with a space instead of the URL-encoded %20.

The parser should ensure all URLs returned are valid and escape any characters as appropriate. This should match the browser behavior of the browser making a request to http://example.com/foo%20bar when a link like <a href="http://example.com/foo bar">foo bar</a> is clicked.