Merge pull request #7 from soniah/df49b4fc0b10ed2cab253cecc8c3d86b72cec41d

prevent panics being passed
Stringer for SnmpVersion - make usage/debug easier
Merge pull request #8 from soniah/7094cc6d90580b526272fac2aa64f9ea636820b5

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
doco on Verax testing
ToBigInt(), uint64ToBigInt()
refactor1 dangling branches, unmarshalGetResponse()
removed dead code - decode()
decodeValue() - set Type correctly (gauge32 tests)
wip verax testing
more detailed debugging - dump bytes, field
decodeValue remove extraneous comments
parseUint() - Counter32, Gauge32