add MAPI over HTTP support

MAPI over HTTP is supported on at lease Exchange 2013 SP1
correct formatting
correct definition

the CAS-Array is the server we want to reach...
do not revert the round-robin patch 1bcfd29
offload the whole auth process onto a backend peer.

with squid-3.2+ you use login=PASSTHRU and *no* Squid auth setup
to offload the whole auth process onto a backend peer.
offload reverse auth to backend
Merge pull request #668 from trendchiller/master