Load Bing images via XHR+blob URL

This allows us access to the byte size of the image.  Then, we can check
the image to discarding only if it matches the size of the "missing"
tile.  This should speed up imagery loading.

This is a work in progress.  It's kind of messy right now.
Clean up image loading via blobs.
Clean up image loading via blob.
Doc improvements.
Fix test failures.
Load all imagery via blobs when using a discard policy.
Fix more test failures.
Update CHANGES.md.
Remove unneeded change.
Remove unneeded requires.
Remove more unneeded requires.
Last cleanup.
Remove the blob after it is no longer needed.
Remove unneeded code.

The entire image becomes elligible for GC a microsecond later, anyway.
Eliminate a bunch of code duplication.
Merge remote-tracking branch 'origin/master' into blob

Conflicts:
	CHANGES.md
Fix typo.
Minify and inline CSS imports.
expose WebGL context options in Scene constructor
Added doc for Scene