pass proxy's scheme to SlipStream service and use for redirect URLs

The SlipStream server currently redirects using a mix of https and http URLs depending on whether the URL was relative (i.e. constructed by the browser) or absolute (constructed by the server).  This was solved previously by rewriting URLs in the proxy, but the server itself should use a consistent base URL (and scheme) for all responses.  This pull request uses the scheme passed in by the proxy and consistently applies this to redirect URLs.  (See issue #177). 

**NOTE**: A related pull request for SlipStreamServerDeps (SlipStream/SlipStreamServerDeps#4) must be applied at the same time.

A side effect of this change is that the URL rewriting is no longer needed in the proxy.  This also now works correctly with the default version of nginx from the EPEL repository.  (A more recent version from the nginx repository was needed for the URL rewriting.)