Reorganize CesiumViewer module configuration.

Flip the baseUrl around so that CesiumViewer is more like a self-contained application which pulls in Cesium modules using path configuration with a Cesium prefix.

Also fix build script to include the Cesium copyright header at the top of the built file.
Merge pull request #2199 from AnalyticalGraphicsInc/cesiumViewerPathCleanup

Reorganize CesiumViewer module configuration.