Fix HealthCheckUtil/MetricsUtil's support for environments other than test and production (#31)

The logic is no longer environment-specific (the environment-specific signatures have been deprecated, and delegate to the default one). In all cases, if there is a registry in the servlet context, it is used.  If not, a fallback registry is instantiated on first access and then used. This supports all cases where the application is running with a properly initialized servlet context, as well as cases (such as testing) where the servlet context is not available.

I've tested the behavior with unit tests, integration tests, dev lightweight and prod lightweight, in Grails versions 2.2.3 and 2.4.3.