Redefined OctetString type as []byte

OctetStrings were being incorrectly based to a Go string.  SNMP
OctetStrings contain binary data.  In many cases they are strings but in
other cases they are binary data such as values containing bit flags.
Casting the data to a string will lead to loss of information (e.g.
nulls).  Instead we'll store the data as a []byte array and consumers
can convert to a string - via string(value) - as required.

Addresses issue #23