Start refactoring Datasource interface

Manually extract all non-dataSourceBrowser specific changes out of the `dataSourceBrowser` branch.
Turn DataSource get methods into ES5 properties.
Merge branch 'master' into dataSource-refactor
Added async option to Model
Fix viewer doc
Do not draw model when scale is zero
Added size in meters/pixels to model
Added ModelMaterial
Added materials to ModelMesh
Model updates
Added color / Cartesian4 interop
Doc and tests for from functions
Tweak allowPicking
More doc
Removed unused czm_pixelSizeInMeters
Add Matrix helper functions
Model updates
Remove unused code
More doc
Added boundingSphere