Update version to b23.
Rename all instances of cesium.agi.com to cesiumjs.org.

Cherry-picked from @kring's d0e34a7a.
Tracking shouldn't be in master.

Also, found a straggler.
Update CHANGES.md
Update CHANGES.md again
Merge pull request #1320 from AnalyticalGraphicsInc/update-address

Update master's references to the website.
Drop MOZ_ WebGL extension prefixes to avoid warnings in Firefox
Changed extensions to use undefined when not available
Added failIfMajorPerformanceCaveat
Workaround tests until failIfMajorPerformanceCaveat is returned by getContextAttributes
Undefined tweak
Added allowTextureFilterAnisotropic
Update CHANGES.md
Added maximumScreenSpaceError
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.