WorkerSolver: Add API for using a custom worker

[gkjohnson]

Something like an encapsulated class that can be instantiated in the worker that can listen to messages so users can write custom logic that listens to and modifies worker state manually.

class RemoteWorkerSolver extends Solver {

    consumeMessage( event );
    onDispatchMessage( event );

}

class WorkerSolver {

    constructor( roots, worker = new Worker( './WorkerSolver.worker.js' ) );

}

WorkerSolverRemote should also be able to use non synced joints so some joints can only live in the worker.

Open Questions

• Should the solver automatically solve indefinitely until a non timeout result is hit? Or should it be up to the user worker to call solve?

[gkjohnson]

This may not exactly be needed. If required users can just pass the joint angles back to the main page to update something like the URDF model in the case they want to perform some kind of special solve in a Worker. A consistent and exposed way to serialize and deserialize the joint graph would be good, though. (Issue #66)