Add VRTheWorldTerrainProvider.
Add VR-TheWorld to Terrain Sandcastle example.

Also fix some problems with terrain providers that aren't immediately
ready.
Fix test failures and update CHANGES.md.
Add tests for VRTheWorldTerrainProvider.
Merge remote-tracking branch 'origin/master' into VRTheWorld
Merge pull request #521 from AnalyticalGraphicsInc/VRTheWorld

Add support for VT MAK's VR-TheWorld terrain server