Register metrics and metrics sets in xml

Closes #40

Registering JVM metrics in XML:
```xml
	<metrics:register metric-registry="metrics">
		<bean metrics:name="jvm.gc" class="com.codahale.metrics.jvm.GarbageCollectorMetricSet" />
		<bean metrics:name="jvm.memory" class="com.codahale.metrics.jvm.MemoryUsageGaugeSet" />
		<bean metrics:name="jvm.thread-states" class="com.codahale.metrics.jvm.ThreadStatesGaugeSet" />
		<bean metrics:name="jvm.fd.usage" class="com.codahale.metrics.jvm.FileDescriptorRatioGauge" />
	</metrics:register>
```