gosnmp.go: Added GetBulk and GetNext functions. Updated SnmpPacket struct with NonRepeaters and MaxRepetitions fields for GetBulk.
marshal.go: Added support for GetBulk and GetNext request types. Renamed unmarshalGoResponse to unmarshalResponse as it handles getnext and getbulk as well.
marshal_test.go: Added packet dumps for getnext and getbulk requests and responses. Unmarshal tests pass for both. Working on enmarshal tests still.