Import UIKit in FLAnimatedImage.h

Since UIImage is referred to, we need to import UIKit. Otherwise we can get build errors about "Unknown type name 'UIImage'".

Moreover, importing UIKit obviates the need to import CoreGraphics in FLAnimatedImage.m.

Fixes #36
Merge pull request #37 from nolanw/fix-imports

Import UIKit in FLAnimatedImage.h