Attemps to add support for multi-folder routes

Hi

 I'm trying to test pages that use requirejs. For some reason, in
 development mode, all my static js client-side files are not under
 the same folder hierarchy ; however I want to serve both hierarchy
 under the same prefix.

 For example, If I have :

    + root
      + public
        + js
          + foo.js
      + deps
        + commons
          + js
            + bar.js

I want to be able to serve :

* File /root/public/js/foo.js as URL '/public/js/foo.js'
* File /root/deps/commonsjs/bar.js as URL '/public/js/bar.js'

I though using 'routes' would do the trick, but unfortunately you can
only use one path.

So I tried to allow specifying routes like this :

    {
      routes : {
        'public' : ['public', "deps/commons"]
      }
    }

The idea being that if a url matches a route, the first file that can
be located in any of the folders passed in the list is returned.

I managed to implement something, but unfortunately, serving the files
becomes incredibly slow with this ; so there must be something very
inefficient somewhere, that I can not find.

I added a server tests, everything passes fine (I had to uncomment the
other 'routes' tests, though.)

So this PR is more about asking for help than anything :)

 * is there a built-in way to deal with my case ?
 * do you have any idea why my patch makes thing so slow ?

Thanks !