manual decodes; request_type, pdu_type, pdu_value fields
marshalVarbind - Integer - rough cut

working up to marshalMsg and TestEnmarshalMsg,
therefore need to refactor Get into Get/Set.
Get/Set/send interface; all TestEnmarshal* passing
rename requesttype/messagetype to pdutype per RFCs
move send() out of gosnmp.go (public interface)