Mercurial access level internal API
This provides the needed parts in the Rails application for SSH support (#11 (closed)).
The central method in
HgAccess will help us keep consistency, and should be applied in
the HTTP case, perhaps later in the Heptapod 0.8 development cycle.
The new internal HTTP API will be called by Heptapod Shell to retrieve the permission and pass it over to the
hg executable that it execs.