extent -> rectangle
Merge branch 'master' into renameExtent

Conflicts:
	Source/Workers/createVerticesFromHeightmap.js
update CHANGES.md
Merge pull request #1612 from AnalyticalGraphicsInc/renameExtent

Rename Extent to Rectangle
Merge remote-tracking branch 'origin/master' into noAlpha

Conflicts:
	Specs/Scene/SingleTileImageryProviderSpec.js