Sync version of JS library

[amarchino]

The commit changes the following files:

• nexus.js Adds a sync Mesh and a sync Instance object that let the user load a complete nexus mesh file at once and then render the object Slight modifications for other objects were made to accomplish the task. Constructors are added to prototypes for runtime detection purposes
• nexus_three.js Adds a sync NexusObject subclass that makes use of the sync Mesh and Instance previously created
• threejs.sync.html An example page showing the usage of the sync mode of the library

This pull request deals with the possible evolution explained in issue #19