First cut at DynamicScene/CZML WallGeometry support.
Merge branch 'master' into dynamicScene-wall-geometry
Merge remote-tracking branch 'origin/master' into dynamicScene-wall-geometry
Merge remote-tracking branch 'origin/issue1603' into visualizer-refactor

Conflicts:
	Source/DynamicScene/DataSourceDisplay.js
Factor out Visualizer interface

It turns out a `Visualizer` instance only needs `update` and `destroy` functions.  Also start cleaning up other visualizer to conform to the interface and remove uneeded cruft.
Merge remote-tracking branch 'origin/master' into dynamicScene-wall-geometry
Start refactoring Datasource interface

Manually extract all non-dataSourceBrowser specific changes out of the `dataSourceBrowser` branch.
Turn DataSource get methods into ES5 properties.
Merge remote-tracking branch 'origin/master' into dynamicScene-wall-geometry
Merge branch 'dynamicScene-rectangle' into dynamicScene-wall-geometry

Conflicts:
	Source/DynamicScene/CzmlDataSource.js
	Source/DynamicScene/DataSourceDisplay.js
Specs and cleanup for DynamicWall.
Add an offset to the grid material.
Merge remote-tracking branch 'origin/master' into grid-offset
CHANGES.md
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