Add Arris DG950A SNMP data extraction module

This module will extract critical data such as WPA and WEP keys from
the Arris DG950a model cable modem via the SNMP protocal.
Instead of bottling pretty much the entire module in one long `if...end`, how about something like

````ruby
if snmp.get_value('sysDescr.0') =~ /DG950A/
  print_good("#{ip} - responded to sysDescr.0 request")
else
  fail_with("#{ip} - not responding to sysDescr.0 request")
end
````ruby

That way you're not over-indented and have a bunch of stair-step `ends` at the end.