Client maintenance

Refactored client code to separate plugin specific code from the core implementation.
Client now contains of a core entity registry, entities which basically perform attribute updates received from the server, scene manager that is still connected to XML3D scene management, and a set of plugin scripts that implement plugin related functions.
Added a simple Event system to allow inter-plugin communication.
Plugin specific function wrappers are now created in the JavaScript class for this plugin.

Things to discuss: The decision to have scene manager still performing updates on an entity's XML3D transformations seems a bit of an arbitrary choice. Transformation updates could move into location.