Add support for a minimum level in TMS sources.

This way the default gdal2tiles.py behavior of only generating tiles up to
the first level that has only one tile will not confuse Cesium.
Use previous behavior when minimum level is no good.
Tests and other polish.
Doc, copy/paste errors, and CHANGES.md.
Remove unused require.
Merge pull request #913 from AnalyticalGraphicsInc/minimumLevel

Add support for a minimum level for imagery providers, especially TileMapServiceImageryProvider