Decoded snmpset, extra fields. Tests failing.

testsEnmarshal_t add:
  * request_type

testsEnmarshalVarbindPosition add:
  * pdu_type
  * pdu_value
marshalVarbind - Integer - rough cut. Tests failing.

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)
readme - Send() beta