Stringer for SnmpVersion - make usage/debug easier
fixed data truncation problem
various fixes on gsnmp.go

* make Conn public, so can do deferred Close() from caller

* actually _return_ error on recover

* denote source of errors

* observe passed-in version

* NewGoSNMP() - port as a parameter, example.go too

* Get() - tidy, handle returned pdu nil
refactor - dangling branches, unmarshalGetResponse(), dead code
ToBigInt(), uint64ToBigInt()
decodeValue() - set Type correctly (gauge32 tests)
parseUint(); detailed debugging; extraneous comments
strings passing; calc_length()
decodeValue() using calc_length - needs more work
verax testing, doco