Provide Easier Way For The NamespaceRegistry To Indicate The Existence Of Namespace

In order to find out if a namespace prefix/URI is registered in the namespace registry, one must either (1) get all the registered prefixes/URIs and loop through them, or (2) query the registry which returns the associated URI/prefix or throws an exception if the prefix/URI is not registered. This is all in accordance with the JCR API.

It might be nice to have a way that indicates if a namespace prefix/URI is registered just by just checking a "boolean" return value. This would be helpful for sequencers, sequencer-support classes, and possibly other parts of ModeShape.