Implement a Ninja Metrics module

If the Ninja Metrics module is present on the classpath the metrics
InstrumentedNinja class will be used instead of the DefaultNinja
class.  This class will log @Timed or @Metered controller methods
in addition to giving metered stats about failed or bad requests.

Additionally, the selected Ninja Cache implementation will be wrapped
by an InstrumentedCache which will log metrics about the cache
activity.

To try this out:

1. `mvn install` in the repository root
2. `mvn ninja:run` in the ninja-servlet-integration-test
3. Startup VisualVM
4. Install the MBeans plugin through the Tools->Plugins  menu
5. Drill into the NinjaJetty instance listed in VisualVM and select the *MBeans* tab
6. Browse the test app and click on the different available *ninja.cache* and *ninja.requests* metrics