Metrics

Initial stab at metrics, more to come later.
 - number of http/net connections & connections by remote address
 - number of http requests (client & server) and requests by uri (only for server)
 - http request latency and rate (client & server)
 - net/http bytes read/written. For http we only record the content/body. So this excludes headers and such.
 - number of event bus & handlers by address
- number of verticles & by verticle type (verticle.getClass().getName()).
When is this JMXReporter ever stopped? It's lifetime should be tied to the lifetime of the Vert.x instance, and when the Vert.x instance is closed it should be closed and all the MBeans unregistered. Otherwise -> memory leak.