Add a bundle to set the correct MetricsRegistry on the ServletContext

It seems that the idiomatic way to pass the desired `MetricRegistry` between libraries is by setting it as an attribute on the `ServletContext` ([see here](http://metrics.codahale.com/manual/servlets/)). This PR does just that, allowing other libraries to contribute metrics to Dropwizard's metric registry (specifically this will be used by [HubSpot/jackson-jaxrs-propertyfiltering](https://github.com/HubSpot/jackson-jaxrs-propertyfiltering) to contribute timing information)

@wsorenson @tpetr