Clean up CZML custom properties demo

[hpinkos]

It took me way too long to figure out what this example was trying to demonstrate

http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=CZML%20Custom%20Properties.html&label=All

• Get rid of the Print values button. I don't think it is particularly useful. If it is important to see the values, it would be better to display them on screen in a label.
• Replace the Use interval data and Use sampled data buttons with a combobox that switches between the two options. Right now neither is seleted by default so nothing is happening as the timeline is scrubbing until you select one of the options
• Change the data to something that is more of a dramatic change. It looks like we were trying to use a real world example, but there is barely enough of a change to see the height of the polygon change. Either represent something different, or replace the PA polygon with an arbitrary polygon with arbitrary height values.