add mrspock to authors
optimise oidToString function to perform better

Before Optiomisation:
BenchmarkOidToString	 1000000	      1077 ns/op

After Optimisation:
BenchmarkOidToString	 5000000	       665 ns/op
mgenov to authors; helper_test license
fix boundary marshalInt16

original author: Marcin Jurczuk (@mrspock)
SNMP SET support for OctetString
tidy SET sanity check