Add Terrain support to BaseLayerPicker

Still needs cleanup.
BaseLayerPicker cleanup

1. Turn on depth test when any terrain other than EllipsoidTerrainProvider is being used
2. All similar Viewer options for terrain that imagery already has.
Ongoing BaseLayerPicker work

Flesh out specs and minor cleanup.
Hopefully final pre PR clean to BaseLayerPicker terrain support.
Put back default imagery and widen the drop down.
Merge remote-tracking branch 'origin/master' into baseLayerPicker-terrain
Tweaks to make Cesium not totally bomb on IE8.
Changes after review.

1. BaseLayerPicker now takes an options object instead of individual parameters.
2. Minor styling fix to dropdown width so that 4 items fit when vertical scroll bar is visible.
3. Add selectedImageryProviderViewModel/selectedTerrainProviderViewModel options to BaseLayerPickerViewModel
4. Fix tooltips for default terrain providers.
Merge remote-tracking branch 'origin/master' into baseLayerPicker-terrain
Merge branch 'master' into baseLayerPicker-terrain
Update CHANGES.md

Remove duplicate b28 entry.
Merge remote-tracking branch 'origin/master' into ie8
Merge pull request #1614 from AnalyticalGraphicsInc/ie8

Tweaks to make Cesium not totally bomb on IE8.
Merge remote-tracking branch 'origin/master' into baseLayerPicker-terrain
Changes after review

1. Fix copy/paste error.
2. Add more information to CHANGES
Merge pull request #1607 from AnalyticalGraphicsInc/baseLayerPicker-terrain

Extend BaseLayerPicker to support terrain
Merge remote-tracking branch 'upstream/master'