code: public static <T> T get(Class<T> pluginClass, T defaultImpl) {
  		return INSTANCE.getInternal(pluginClass, defaultImpl);
  	}
docs: Returns an instance of pluginClass which is guaranteed to be identical throughout
  the runtime existence of this library. The returned instance is determined by:
  <ol>
  <li> the first call to {@link #register}
  <li> if a system property named <code>durian.plugins.{pluginClass.getCanonicalName()}</code> is set to
  the fully-qualified name (<code>Class.getName()</code>) of an implementation class with a no-argument
  constructor, then an instance of that class will be instantiated and used as the plugin implementation
  <li> the <code>defaultImplementation</code> that was specified in the first call to get()
  </ol>
  @param pluginClass The interface which is being requested.
  @param defaultImpl A default implementation of that interface.
  @return An instance of pluginClass, which is guaranteed to be returned for every future request for pluginClass.