HTTP Basic-Auth support for direct access to GitBlit plugin.

Allow to access directly to GitBlit without going through
the standard Gerrit login process interactively: username
and password are retrieved using HTTP Basic-Auth from the
Http Request.

This is useful when using GitBlit RSS feeds that cannot
authenticate using the manual Gerrit form-based
authentication.

Change-Id: I3f31fdb362e9ed60a4acacebb2be9776171ea4ff
Signed-off-by: Luca Milanesio <luca.milanesio@gmail.com>