Use exact matches in asset routes

The previous way mixed the js/css routes, so that a request for
a assets/js/css_name.js and assets/css/js_name.css would be
matched.

I'm not sure it's possible to use a single regexp to match
against both while still having the correct captures yielded,
so just use two regexps in an array.

This removes the %242E -> . conversion and reconversion.
I don't think we should support having assets outside the
css_folder and js_folder options.  Instead, the css_folder
and js_folder options should point to the top level directory
containing all of the assets. There are more changes
necessary to implement that, though.