Support for CZML defined clocks.

This is the client implementation of the CZML clock properties defined in [this](https://github.com/AnalyticalGraphicsInc/czml-writer/pull/46) already merged czml-writer pull request.

So far this object is a little atypical (kind of like availability).

1. Clocks are only supported on the special `document` object (defined by an object with that id).
2. Clock properties are constant.  They can change when new settings come in, but they are not dynamic properties.
3. Regenerated simple.czml to include a clock.
4. Modified CesiumViewer to use the clock object if available, or fall back to the old detection if not.

As an aside, my initial implementation of CZML is showing some cracks in the foundation.  I really want to refactor the whole DynamicScene level some time soon and add official support for streaming. (but all of that is obviously outside the scope of this simple pull).