Added 802.11n rate handling

Added handling of 802.11n rates. To work out that actual transfer rate,
you need to know the guard interval and bandwidth, which I can’t find
from snmp :-( Hence, this uses the m0-m15 index for the non legacy
rates.

http://www.cisco.com/c/en/us/td/docs/wireless/access_point/12-4_10b_JA/c
onfiguration/guide/scg12410b/scg12410b-chap6-radio.html#wp1017196

http://www.oidview.com/mibs/9/CISCO-DOT11-ASSOCIATION-MIB.html