GIF input support

Hello, 

I've written the input support for gif images. I used [giflib library](http://giflib.sourceforge.net/), which is very mature and already used by qt. 

A few things to point out:
* It can be built against giflib 4 (current stable) and 5 (newest) and hopefully earlier versions.
* Animated gifs are presented as sets of subimages.
* I also updated the documentation. Please check if openimageio.pdf is generated correctly.
* Some gif images are interlaced, ie. their scanlines are ordered differently than normal ([I've put an illustrative example here](http://mszczepanczyk.github.io/gif-interlacing-example/)). Because scanlines can't be randomly accessed read_native_scanline function caches the part of the image that is already read. Otherwise, without caching, a simple consecutive scanlines reading would end with **a lot** of unnecessary rereadings and file reopenings. However, I don't know if the approach I used is proper. Please check the code.

Any comments are appreciated :)

Thanks,
Mariusz