Add support for Extreme

[Ondjultomte]

from nelsnmp.hostinfo.device import HostInfo
from nelsnmp.snmp import SnmpHandler

handler = SnmpHandler(host='itb-3206-a', version='2c', community="public")
hostinfo = HostInfo(handler)
hostinfo.get_all()
hostinfo.dict
{'sysobjectid': '1.3.6.1.4.1.1916.2.93', 'vendor': 'UNKNOWN', 'description': u'ExtremeXOS (Stack) version 15.3.1.4 v1531b4-patch1-33 by release-manager }

Vendor: Extreme
OS: ExtremeXOS
Model: Summit (But this is not obvious from the output)
Version: 15.3.1.4 v1531b4-patch1-33