Solr output

This is hopefully a fix for https://logstash.jira.com/browse/LOGSTASH-1405

It's an output plugin that sends logs to Solr via HTTP, much like elasticsearch_http does. Similarly to that one, the options are to choose the Solr URL, the flush size and idle flush time, and the document ID.

It adds rsolr as a new dependency, which is the library used for talking to Solr. It's nice because you can send hashes as documents which makes the "flush" method pretty straightforward. For buffering, "stud/buffer" is used, and for automatic ID generation "uuidtools".