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() - allow port to be passed in as a parameter

* Get() - tidy, handle returned pdu nil
doco on Verax testing
ToBigInt()
wip verax testing - Gauge32
refactor1 dangling branches, unmarshalGetResponse()
removed dead code - decode()