Fix sensu data source reporting errors when using a server filter on a working server

Ok, here's another one for you :)

The previous implementation of the sensu data source would return an error if you specified a server filter but the event list didn't contain any events from that server (master on line 60). In this state you actually want a "green" result because it means there are no errors on that server.

I did some refactoring of the code as well.  I'm not a Ruby programmer so I have a feeling there are some non-Ruby-ific style in there.  Also, I tried to test all of the cases and I think they all work (e.g. with/without server filter, with/without event filter, etc.).

Give this one a thorough review before accepting it.