Add support for ?replace=true query option.

Add support for ?replace=true query option. This option will bypass the
check to see if the thumbnail exists in CEPH, generate the thumbnail,
then store it back causing an existing thumbnail to be replaced.

This will be useful for cases where a thumbnail is improperly generated
or broken.
Merge pull request #33 from Wikia/purge

Add support for ?replace=true query option.