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
Merge branch 'master' into multiPicking

Conflicts:
	CHANGES.md
Fixed pick
Merge pull request #978 from gbeatty/multiPicking

Added `Scene.drillPick` for picking of multiple primitives