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


Consider using the Apache Commons Math library rather than writing your own Statistics library. For example, http://commons.apache.org/proper/commons-math/javadocs/api-3.2/org/apache/commons/math3/stat/descriptive/DescriptiveStatistics.html will calculate mean, standard deviation, and variance for you.