[DO NOT MERGE] Add Request URL Rewriting capabilities

The main client for this are Tizen Web Applications. These apps will
reference certain resources (e.g.  Tizen jQuery libraries) in their HTML
files without passing their full path; only a certain prefix
("tizen-web-ui-fw/", for example) is passed to the "src" attribute of
"script" tags.

URL rewriters are functions that will be called whenever there's a match
with a certain prefix, allowing platform-specific code to be called to
deal with their intricacies.