Added SNMP Trap input

# SNMP Trap Input

added input,  included snmp gem in gemspec.

config look something like -

```
input {
  snmptrap {
    type => "snmptrap"
    port => 162
    community => "public"
  }
}
```

the object from the snmp library is pretty sucky so I elected to just .inspect it for the event @message contents.     next version can try to extract the object constituents into fields.

@message will look like

```
#<SNMP::SNMPv1_Trap:0x2635a891 @varbind_list=[#<SNMP::VarBind:0x1ef132e1 @value="teststring", @name=[1.11.12.13.14.15]>], @timestamp=#<SNMP::TimeTicks:0x62bfcfad @value=55>, @generic_trap=6, @enterprise=[1.2.3.4.5.6], @source_ip="127.0.0.1", @agent_addr=#<SNMP::IpAddress:0x1edb9981 @value="\xC0\xC1\xC2\xC3">, @specific_trap=99>
```