Adding metric tracking framework for Azkaban

This change includes:-
1. Metric Tracking framework under Azkaban.metric package
2. Default metric reporting emitter under Azkaban.metric.inmemoryemitter
3. Exposed metric functionality /stats using statsservlet
4. Number of running flows and jobs as metrics

We will also have a curl option for following:-
* To set tracking interval for each metric
* To enable and disable metric tracking in Azkaban
* To set metric history details like maximum tracked interval and display points for /stats servlet


You `synchronized (metric)` above, but you are using `metric` inside a new thread here, so the synchronization lock does not apply to this thread. See http://stackoverflow.com/questions/15926400/execution-of-new-thread-inside-a-synchronized-block for details.