issue132 don't check returned OIDs are increasing

CHANGELOG

changelog

changelog

changelog

exponential timeout + doco; goimports

marshalInt32()

go 1.11 - travis

Support SNMPv3 Encode/Decode (gosnmp#160)

* Store MsgMaxSize, support SNMPv3 SnmpDecodePacket
* Export MsgMaxSize
* Add SetMsgID/SetRequestID, validate Encode/Decode

Merge 'gosnmp#158 Add mock for Handler interface'