update project to use logfmt (https://github.com/cyberdelia/logfmt-ruby)

The key/value parsing in the logfmt package is more robust than the current implementation.  In addition, there is a [patch](https://github.com/cyberdelia/logfmt-ruby/pull/5) to the logfmt project that will offer even greater support for integer and float values.  This update will make sure that non-quoted integer and float values are returned as Integer and Float objects respectively.

This feature of having `stat_num_items_processed=234` return an Integer value of `234` is necessary in order to have type aware data stores such as ElasticSearch be able to correctly [calculate aggregate values](http://www.elasticsearch.org/guide/en/elasticsearch/guide/current/aggregations.html).  If string values get persisted and you try to compute an aggregate, you end up getting a `ClassCastException[org.elasticsearch.index.fielddata.plain.PagedBytesIndexFieldData cannot be cast to org.elasticsearch.index.fielddata.IndexNumericFieldData` error.  Since [MessagePack](http://msgpack.org/) supports integer & float types, we should try our best to persist that type from the original key=value format to the target data store.

I've tested this update along with the 0.0.4 update of logfmt and have been able to successfully persist into ElasticSearch integer and float values with their proper types.

Thanks