Support animated gif in meme

Summary:
Use imagemagick convert to first detect the number of gif frames,
then +adjoin to split them in tmpfile, then call applyMemeTo to
each frame, then convert them back.

Test Plan:
Add memes in sandbox
with both gifs and non-gifs. Note that gifs are quite slow.

CC: aran, epriestley, chad

Differential Revision: https://secure.phabricator.com/D5821