Implemented using corbett's suggestion in issue #525. Uses
Flipboard/FLAnimatedImage in an AttachmentAdapter. Detects gifs using
new Animated category in MIMETypeUtil.
Backwards compatible with previous versions of Signal on iOS for both
sending and receiving Gifs, though they are sent/received in older
versions as UIImage and won't animate. Gifs also animate on both ends of
conversations with TextSecure users on Android.
//FREEBIE
Matthew Chen
Michael Okner
Christine Corbett
Frederic Jacobs