Add in new mod_proxy "extension" module: mod_proxy_express.

This is a module developed by myself to support the
RedHat OpenShift Express project, to allow for dynamic
and fast reverse-proxy switching.

The idea is to easily map Host: header entries to
backend servers, with the mapping done in a DBM file
(think of mod_vhost_alias, kinda, for mod_proxy).
Yeah, mod_rewrite and RewriteMap could kinda do
it but (1) that is slow and (2) no support for
ProxyPassReverse capability.

Donated to the ASF.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1102733 13f79535-47bb-0310-9956-ffa450edef68