handle zero-result queries, specify index/type, add stub of rspec

Attempting to aggregate several PRs, feature requests and issues:

https://github.com/elasticsearch/logstash-contrib/pull/57
https://github.com/elasticsearch/logstash-contrib/issues/95
https://github.com/elasticsearch/logstash-contrib/pull/96

with help and input from @greem @sisutcliffe @bleskes et al.

Should we support sprintf() values?

```index: event.sprintf(@index), type: event.sprintf(@type),...```

?