Added `Scene.drillPick` for picking of multiple primitives
handle multiple ways of hiding primitives
changed test to check if setShow function is defined
fixed doc comment
Merge branch 'master' of https://github.com/gbeatty/cesium into multiPicking

Conflicts:
	CHANGES.md
	Source/Scene/Scene.js
Merge branch 'master' into multiPicking

Conflicts:
	CHANGES.md
fixed bad merge
code cleanup
sandcastle drill-down picking example
initial support for geometries
removed unused require
Merge branch 'master' into multiPicking

Conflicts:
	CHANGES.md
correctly handle geometry picking
Merge branch 'master' into multiPicking

Conflicts:
	Apps/Sandcastle/gallery/Picking.html
	CHANGES.md
added PickSpec.js
cleanup and additional tests
Initial commit for issue #1076.  Added translucencyByDistance to billboard
and incorporated into Label.  SandCastle examples added for both label and
billboard fading based on camera distance.

Signed-off-by: Alex Wood <abwood@gmail.com>
Merge remote-tracking branch 'agi/master' into issue1076
Fixed typo in Billboard asset name.

Signed-off-by: Alex Wood <abwood@gmail.com>
Fixed bug that would occur if some billboards in a collection did not use
scale/translucencyByDistance, their default values would cause issues.
For translucency and scale, we should default to 1.0 which results in an
identity transformation (e.g., scale by 1).

Signed-off-by: Alex Wood <abwood@gmail.com>