Change User and RemoteInstance objects to be asynchronously constructed

User and RemoteInstance objects had been using storage.load within their constructors.  Because storage.load is an async function, this resulted in User objects in the network.roster and RemoteInstance objects in the user.instances_ array that were not fully ready for use.

I've changed both User and RemoteInstance objects to have a static .create method, which acts as a factory, and returns Promises that get fulfilled when the User/RemoteInstance objects are fully ready for use.  This way we will no longer have any partially constructed Users in network.roster or RemoteInstances in user.instances_, and don't need public variables like isReadFromStorage.

This also fixes https://github.com/uProxy/uproxy/issues/721

Tested in Chrome, Firefox (UI still loads), updated "grunt test" to reflect new async behavior
I'm removing this rather than commenting it out.  This test no longer works quite right with the new behavior.  The problem with the way it was written is that the line spyOn(user, 'handleClient') was only executed after network.handleClientState - and by that point user.handleClient had already been called so it was too late to spy on.

I also think the way this test is written now is better.  All we really want to test is that a user gets added when handleClientState is called with a userId not in the roster.  Checking that user.handleClient has called is more of an implementation detail and doesn't actually check on the behavior of our code.