Accept an optional format in rocketfuel image URLs (bug 944109)

There is some additional cleanup, doc and tests but the heart
of the issue was making collections image use a ".png" format
when an image URL is returned, but also let us PUT/DELETE to
the format-less URL.
Merge pull request #1547 from diox/fix-api-PUT-collections-image

Accept an optional format in rocketfuel image URLs (bug 944109)