Initial submit for pixel offset scaling by distance.
Added pixelOffsetByDistance to labels.  Added Sandcastle example to demonstrate a usecase.
Updated Dynamic layer to support time dynamic Label and Billboard PixelOffsetByDistance.
Refactored PixelOffsetByDistance to instead be PixelOffsetScaleByDistance.  This required the addition of a private member, Billboard._translate that is used by labels to separate glyph translation from pixelOffset settings.
Updated Billboard.computeScreenSpacePosition to use _translate in addition to pixelOffset for screen space positioning.  Updated LabelCollection Unit tests.
Updated DynamicScene to use the pixelOffsetScaleByDistance over the former pixelOffsetByDistance.  Added BillboardCollection, DynamicLabel, and DynamicBillboard unit tests.
Updated Changes.MD.  Minor cleanup.
Added Billboard.setPixelOffsetScaleByDistance SandCastle example.  Minor tweak to Label's pixelOffsetScaleByDistance sandcastle example.
Minor cleanup.
Updated Billboard sandcastle example.
Merge remote-tracking branch 'agi/master' into offsetByDistance

Conflicts:
	Apps/Sandcastle/gallery/Billboards.html
	Apps/Sandcastle/gallery/Labels.html
	CHANGES.md
Minor cleanup.
Merge pull request #1324 from abwood/offsetByDistance

Offset by distance
Fix code highlighting in CHANGES.md
Handle CZML availability with multiple intervals.

`DynamicObject.availability` is now a `TimeIntervalCollection` instead of a `TimeInterval`.  The availability property is still always replaced when a new availability is provided, no merging happens.
Merge remote-tracking branch 'origin/master' into dynamicObject-availability

Conflicts:
	CHANGES.md
Update CHANGES.md

`DynamicObject.availability` update is a breaking change.
Remove unecessary if checks.
Fix whitespace.
Remove duplicate function call.