Improvements to WebMapServiceImageryProvider

Allow user to specify tile width and height, minimum level, and tiling
scheme.  When the tiling scheme is WebMercatorTilingScheme, tiles are
requested in EPSG:3857 instead of EPSG:4326.

Also changed the `rectangle` constructor parameter to affect only the
provider's rectangle, not the tiling scheme's rectangle.  This means that
provider's with a limited extent will still have tiles that line up with
the terrain tiles (assuming GeographicProjection), which will result in
better performance.  It's possible to get the old behavior just by
constructing and passing a tiling scheme with limited extent to the
constructor.
Update CHANGES.md.
Merge remote-tracking branch 'origin/master' into wmsImprovements

Conflicts:
	Source/Scene/WebMapServiceImageryProvider.js
Remove unnecessary code.
Merge master to wmsImprovements
Merge pull request #2061 from AnalyticalGraphicsInc/wmsImprovements

WebMapServiceImageryProvider improvements