Add support for Riverbed Steelhead and Stingray/Steelapp ZeusTM based appliances

Content is ready and tested against software and hardware devices.

Updated STEELHEAD-MIB and new ZXTM-MIB-SMIv2 will be added.

[user@box ~/SRC/observium-trunk-development]$ svn status
M includes/definitions/sensors.inc.php
M includes/discovery/os/linux.inc.php
A includes/discovery/sensors/steelhead-mib.inc.php
A includes/polling/os/steelhead.inc.php
M mibs/riverbed/STEELHEAD-MIB
A mibs/riverbed/ZXTM-MIB-SMIv2
[user@box ~/SRC/observium-trunk-development]$