Swift port of FLAnimatedImage with updated demo

This is a straightforward port of FLAnimatedImage to Swift, re: https://github.com/Flipboard/FLAnimatedImage/issues/14.

** Requires Xcode 6.1 or higher to compile **

The main classes are AnimatedGif and AnimatedGifView (same idea as FLAnimatedImage and FLAnimatedImageView). I added an additional screen and did a little bit of refactoring to the demo project to allow you to run either the Objective-C version or the new Swift version, which turns out to be a pretty good test to make sure they are as close to interchangeable as possible. We are using this in production on the Weebly iPad app and it is at a pretty stable point now.

The main issue is what to do since some people may not want to add Swift code to their codebase, and it also won't work with cocoapods yet. Some projects such as Masonry/Snappy, AFNetworking/AlamoFire are basically breaking the swift port into a separate library so that would also be an option, but since this is a port of the work here I wanted to put it up for discussion to see what you all think.

- @dkasper, props @ketzusaka and @jberkman for code review