Add dynamic resizing to Cesium Viewer layout

Use media queries to provide 3 different configurations based on screen width.  We still need to manually place the centralBody logo because it's currently not stylable by CSS. This makes the widget look better when embedded or on smaller resolution screens.

Hopefully everyone likes the sizes I picked, but if not, please suggest some new ones.
Change resize bounds from 1200 to 900.
Merge pull request #533 from AnalyticalGraphicsInc/dynamicResize

Add dynamic resizing to Cesium Viewer layout