Use RGB textures for imagery without alpha.

Currently only Bing imagery claims to not have alpha.
Update CHANGES.md.
Merge remote-tracking branch 'origin/master' into noAlpha
Merge remote-tracking branch 'origin/master' into noAlpha

Conflicts:
	CHANGES.md
Add specs for hasAlphaChannel property.
Merge remote-tracking branch 'origin/master' into noAlpha

Conflicts:
	Specs/Scene/SingleTileImageryProviderSpec.js
Merge pull request #1590 from AnalyticalGraphicsInc/noAlpha

Optimization for imagery providers without alpha channel