add activemq_xml agent check

I wrote a quick ActiveMQ agent check based on the xml queue status page, e.g. `http://user:pass@localhost:8161/admin/xml/queues.jsp`

It will emit the following gauges for each queue with the tag `queue:<queue-name>`
* `activemq.queue.consumer_count`
* `activemq.queue.dequeue_count`
* `activemq.queue.enqueue_count`
* `activemq.queue.size`

And an additional gauge `activemq.queues.count` which is the total count of queues.