Cache the result of the CDN URI.

This doesnt change after the first time you get it so lets cache it. This also
removes the network request on each file. This can be a issue where you dont
have caching turned on eg; the admin.
Make cache keys unique.
Just use the deafault ContentFile.

This will just use the default ContentFile for the storage object. We actully
dont need to change the way the storage file works. Since the new pyrax client
will return the bytes we need through _get_object(name).get() || .fetch.
Fixed the swiftclient headers. It now properly sets headers.
Updated readme before release.