Added the gifify function.

This allows to convert a .mov file into an animated GIF file. Based on
these resources:
* https://gist.github.com/SlexAxton/4989674
* https://gist.github.com/paulirish/b6cf161009af0708315c

Usage: 
* `gifify foo.mov` will convert the `foo.mov` file into the animated file `foo.gif`, preserving the original screen size.
* `gifify foo.mov 600` will convert the `foo.mov` file into the animated file `foo.gif`, rescaling to 600 pixels in width.