Allow registration of sequencer objects at runtime

We are using ModeShape in an OSGi environment. Our sequencers are registered dynamically during runtime. At the moment the only way to do this is to manipulate the configuration. The sequencers are registered by supplying their class name.

It would be nice if sequencers could also be registered during runtime using objects created by clients. This would allow more flexible sequencers, e.g. wrapped by a proxy which evaluates custom annotations.