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


Yes, this we have removed this emitter due to some issue with the way gmetric display graphs. I have kept it if we ever decide to move to ganglia.