Velocity support

This is a first cut at adding velocity support that goes along with AnalyticalGraphicsInc/cesium#1790.  We chose to do a new `cartesianVelocity` property instead of specifying velocity separately.  There are 3 reasons for it.

1. It makes the CZML more readable since everything is in one place
2. It makes things more compact since it doesn't require the time values to be replicated.
3. It enforces the fact that there needs to be a one to one mapping of position samples to velocity samples.

That being said, I'm open to suggestions.  Another idea would be to let any sampled property specify derivative information via sub-properties.