Adding mib2rb utility

 - mib2rb is just like mib2c, only it uses ERB templates instead of the mib2c DSL
 - Added descendants method to node class
   + Makes writing ERB template for mib2rb much easier
 - Now defaulting OID sub-id size to sizeof(ulong) if Net::SNMP isn't initialized
   + This should work for the vast majority of cases, since this is the default for netsnmp now
   + To actually determine the sub-id size, Net::SNMP needs to be initialized, which some
     performance critical apps may not want to do.
   + A warning is logged when this defaulting happens, which tells how to fix the problem.
   + May want to consider letting client set this value if desired.