Stop mixing SNMP::BER into root namespace

Previously, anyone who made use of ruby-snmp would find that SNMP::BER would end up being mixed into the root namespace (really, Object). This is generally a big no-no, unless it is very clear that the mixin is intended to extend every object within the ruby VM (and I don't believe that this is what has been intended with SNMP::BER).

This pull request modifies things so that the SNMP::BER methods are presented to only those classes that actually use them.