LUN-791 -- Thumbnails can also be served from the CDN if they're older than the CDN invalidation time
LUN-791 -- Serve thumbnails from the cdn (various improvements)

* a more elegant implementation for the CdnAwareThumbnailFile decorator
* filer.utils.cdn.get_file_url_from_cdn can now cope with file objects
  that don't have timezone aware datetime fields
* add an unit test
LUN-791 -- Rename get_file_url_from_cdn to the more suggestive name get_cdn_url
Removed extra blank line (PEP-8)
Merge pull request #14 from pbs/LUN-791

Lun 791 -- Thumbnails can also be served from the CDN if they're older than the CDN invalidation time