[metrics] Adds persistent metrics to Chronos.

CHANGELOG
=========
* Slight `pom.xml` cleanup
* Updated `Main` to have dropwizard manage the lifecycle of the metrics
  reporter and include `JobMetricsModule`.
* Marked `DescriptiveStatisticsSerializer` as deprecated.
* Added `HistogramSerializer` to provide metrics.
* Update `JobManagementResource` to reflect new `JobMetrics`.
* Added `JobMetricsModule` which uses callbacks from Dropwizard to
  manage the lifecycle of the metrics reporter and inject
  `MetricsRegistry` and `JobMetrics` where necessary.
* Updated `MainModule` to include extra parameter.
  **NOTE**: We probably can just
  `bind(classOf[JobScheduler]).in(Scopes.SINGLETON)` rather than going
  through all the rigmarole of simply passing parameters on.
* Updated `SchedulerConfiguration` to reflect ability to persist metrics
  to Ganglia.
* Slight spelling correction in `ZookeeperModule`
* Created `JobMetrics` to abstract away metrics-related bookkeeping for
  jobs.
* Updated `JobScheduler` to utilize new `JobMetrics`.
* Removed `DescriptiveStatistics` from `JobUtils`
* Updated all tests to reflect new parameter for `JobScheduler`
* Updated `config/README.md` to reflect new options accepted.