Alias image_exists to image_identifier_exists
Alias image_exists to image_identifier_exists

Since we no longer use the client side md5() as the image identifier in Imbo by default,
image_exists is more suitable as an alias for image_identifier_exists. If you need the
old functionality, add hashing before calling the library yourself.

imbo/imboclient-python,imbo/imboclient-python,imbo/imboclient-python