api: proper GIF resize

[omelkonian]

• Adds custom behaviour for resizing GIF images, as PIL.Image.resize
  does not work out-of-the-box (addresses Operations on GIF images do not preserve its frames. #41).

• Additionally passes save_all=True as a keyword arguement on
  PIL.Image.save when serving the file over HTTP (addresses Operations on GIF images do not preserve its frames. #41).

• Bumps Pillow's version to 3.4, where they introduce functionality
  for creating GIF images out of individual frame images.

• Updates tests to cover GIF resizing.

Signed-off-by: Orestis Melkonian [email protected]

[drjova]

@omelkonian could you please have a look on those issues?

flask_iiif/api.py:33 in public class `MultimediaImage`:
        D300: Use """triple double quotes""" (found """-quotes)
flask_iiif/decorators.py:46 in public function `api_decorator`:
        D401: First line should be in imperative mood; try rephrasing (found 'API')
flask_iiif/__init__.py:92 in public method `init_restful`:
        D401: First line should be in imperative mood; try rephrasing (found 'Setup')

[omelkonian]

@drjova ping