Fix docs for metrics-servlets.

Closes #412.
Clarified documentation on context listener.

MetricsServletContextListener does not instantiate any registries of its own, it merely injects provided registries into the servlet context.
Change the timing for ScheduledReporterTest.

Hopefully some improvement for #413.
Merge pull request #414 from TomRK1089/master

Clarified documentation on MetricsServletContextListener.
clarify the VisualVM plugin to use for viewing JMX MBeans
Merge pull request #415 from zathomas/master

clarification on the VisualVM plugin to use