Add support for a minimum level on OpenStreetMapImageryProvider
Update CHANGES.md
Check the number of tiles at the minimum level
Merge pull request #1409 from andre-nunes/minLevel

OpenStreetMapImageryProvider can now make use of minimum level
Minor tweaks to OSM imagery provider.
Merge pull request #1422 from AnalyticalGraphicsInc/minorOsmTweaks

Minor tweaks to OSM imagery provider.
Merge remote-tracking branch 'origin/master' into gltf