Make the Server header reflect the backend name and version
fix handling of schema atoms for yaws
additional changes to support alternative web servers

* Let mochiweb utilities handle the urlsplit_path/1, parse_qs/1, and
  parse_cookie/1 functions, rather than making each web server supply those
  utilities.

* Change yaws code to use a dispatcher rather than an appmod.

* Various code cleanups.
Add wrq:req_headers_to_list to expose headers in a normalized form

This avoids the need for something like
mochiweb_headers:to_list(wrq:req_headers(RD)), which breaks the backend
abstration.
use new Yaws merge_header capability

To properly handle multi-value headers, especially Set-Cookie headers, use
the new Yaws yaws_api:merge_header function.