Credit manager

Added new objects `Credit` and `CreditManager`.

The credit manager is responsible for adding and removing credits to an HTML element that the constructor takes as a parameter.  Every imagery provider and terrain provider has a `Credit` that can be retrieved using the function `getCredit()`.

A credit manager can be created by the Scene when a `creditContainer` is passed into the constructor.  This has to be passed to the CentralBody constructor in order for it to be used. (`var cb = new CentralBody(ellipsoid, scene.getCreditManager());`)  If no credit manager is passed to the central body, a default one is created.

The central body tells the credit manager to update the visible credits during it's update function.

I don't think this will break any existing code, unless it calls `getLogo()` on an imagery or terrain provider.