first pass at terrainSample contrib
fix formatting issue
Enable terrain depth testing.
Fix JSHint warnings, and other minor cleanup.
Refactor terrainSample function.

Mostly I moved some of the interpolation functionality into
`HeightmapTerrainData`, and used some fancy features of the promise
library to avoid manually tracking tile completion.
place marker closer to everest peak
Put bottom of billboards on terrain surface.
Merge remote-tracking branch 'origin/master' into terrainSample
Merge remote-tracking branch 'origin/master' into terrainSample
Add optional request throttling to other terrain providers.
Doc and minor improvements to terrainSample.
Rename terrainSample to sampleTerrain to make it more verby.
Clean up terrain example a bit.
Add tests for samplerTerrain.
Add interpolate method to TerrainData, and update CHANGES.md.
Fixes #568
Merge remote-tracking branch 'origin/master' into terrainSample

Conflicts:
	CHANGES.md
Improve doc and add example for sampleTerrain.
Merge remote-tracking branch 'agi/master' into terrainSample
Merge remote-tracking branch 'origin/terrainSample' into terrainSample