Credit manager

Added new objects `Credit` and `CreditDisplay`.

The credit display 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 display is created by the Scene and passed to the FrameState.  A default `creditContainer` is created in the CreditDisplay constructor when one isn't provided to the scene.

The central body tells the credit display 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.