add reverse proxy support

Add support for a reverse proxy that handles multiple domains via host name
as prefix in the request URI (https://ssl-proxy.tld/domain.tld/).

As the reverse proxy is transparent for the web server the REQUEST_URI and
SCRIPT_NAME need manual adjustments. This patch replace the direct use
of this _SERVER variables with function calls and extend this functions
to overwrite the webroot. Additionally it adds a Sabre request backend
with extends the Sabre_HTTP_Request to use the same functions.