snmptrap input,  added snmp to logstash.gemspec

# snmptrap input

uses the snmp gem to parse snmptraps into a string.       I had some trouble trying to get the actual object created by the snmp gem into fields so I opted to just .inspect the object and get a string to whack into @message.     grok/filters can clean it up if needed.

might do another pass when I get better at this ruby thing to fill the fields instead.