Make 'if "something" not in [nonexistent] { ... }' succeed

My argument is checking for the absence of a value in a field that doesn't
exist should return true, currently it is false.

This fixes the case where an event which has never been tagged now works with
this logic:

    grok {
      ...
    }
    if "_grokparsefailure" not in [tags] {
      ...
    }
Small enhancements to netflow codec

- Add configurable target field for netflow fields, defaults to "netflow"
- Add configurable filter to restrict which versions to accept
Merge pull request #682 from bodgit/notin

Make 'if "something" not in [nonexistent] { ... }' succeed
Merge pull request #686 from bodgit/netflow

Small enhancements to netflow codec
Merge git://github.com/logstash/logstash