Update stale comment.

We're OK with always initializing it since it seems cheap and we already have weak references to the object elsewhere (e.g. `-addFrameIndexesToCache:`), so the weak system will need to clean up anyway.
Another attempt to fix occasional crash on memory warning race condition.

This builds on top of the safety logic introduced in 523f1d600d17f65961d1746a00c7ccc7dc7ac128 but seems like having a better chance at reliably working.
Move to #import single header

Currently both headers, FLAnimatedImage.h and FLAnimatedImageView.h need to be imported. It seems it would make sense to just make it one.

See https://github.com/Flipboard/FLAnimatedImage/issues/8

Also updated README with simpler instructions on how to get started.