Improvements in backends.snmp.OpenWRT + tests
Implemented following methods:
 * RAM_total
 * to_dict()
os method for snmp.AirOS + test
get_interfaces method for snmp.AirOS + test
to_dict method for snmp.AirOS + test
Added test_name in tests.snmp
Improvement to model method for snmp.AirOS + test
manufacturer method for snmp.AirOS + test
firmware method for snmp.AirOS + test
Added test_uptime in snmp.AirOS
Added test_uptime_tuple in snmp.AirOS
Added methods in to_dict smnp.AirOS