add multiple domains reverse SSL proxy support

Add support for a reverse proxy that handles multiple domains via different
web roots (https://proxy.tld/domain.tld/owncloud) and only forwards SSL
connections unencrypted to the web server.

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
that extends the Sabre_HTTP_Request to use the same functions. It allows
to detect the reverse proxy via regular expression for the remote IP
address and conditional overwrite the host name, protocol and web root.