Added "consumer_lag", "lo_offset", "hi_offset" to stats output.

Enabled by default in consumer mode if 'statistics.interval.ms' is non-zero
Added syslog-ng, collectd and fuse_kafka to the list of users
Drop earlier (unwanted) messages from fetched messagesets
Added more metrics to statistics

 * toppar: outbuf_msg_cnt
 * toppar: waitresp_msg_cnt
 * handle: name
 * handle: type
 * handle: msg_cnt
 * handle: msg_max
Dont require auto.commit.enable for stored offsetting (issue #171)
Enabled multi-OS builds for travis-ci