Adding filters for WxS requests (Porting back PR 1081 from official logstash repository)

Following PR https://github.com/logstash/logstash/pull/1081: taking into account the remarks then porting it back here.

I am not sure on how to add the needed gem as dependency nor to use the provided makefile in this repository. I developed / tested onto the logstash repo, then porting it back into this one.

Cheers,