Replica set support for the MongoDB ouput

I'm not sure if I've done this right. Anyway here is a pull request for the Replica Set support in the MongoDB output for Logstash.
I'm not sure how you update your website or the documentation for the output, but I put some comments into the module. Ruby not my strong point so please feel free to rewire.

Tested using RS and it works a treat.

Regards: https://logstash.jira.com/browse/LOGSTASH-626