Added support for <metrics:reporter ../> bean declaration.

Modified existing JmxReporter to support new bean parser.
Updated readme to include information on metrics:reporter

Modified generic reporter to use default metrics registry if none is
provided.
Modified "metrics-registry" option of <metrics:jmx-reporter /> to be optional
Merge remote-tracking branch 'dealer/feature/metrics-reporter-tag' into 2.x

Conflicts:
	src/main/java/com/ryantenney/metrics/spring/config/JmxReporterBeanDefinitionParser.java