adding options for animated gif capturing (Mac only via wacaw)

Using [wacaw](http://webcam-tools.sourceforge.net) and [ffmpeg](http://www.ffmpeg.org) (and ImageMagick's convert) it is possible to generate and annotate a short animated gif for your lolcommit.  

Use -a 3 or --animate=3 option in your `.git/hooks/post-commit` call to capture and animate (for 3 second gif) OR set the `LOLCOMMITS_ANIMATE` environment variable

Some extra points to note;

* **currently mac only**
* the longer the time, the larger the animated gif file-size
* if `LOLCOMMITS_ANIMATE` is not set (or set at 0) - lolcommits will operate as normal
* defaults to a 320x240 sized animated gif (reasonable dimensions to keep file-size down)
* more info on wacaw is available here; http://webcam-tools.sourceforge.net

I've tested with a few of the plugins and it appears to work OK! Also I updated the README.md with instructions on how to enable and configure.

Any issues let me know.  I'm happy to answer any questions, or improve the PR in any way.  I'll probably blog about my adventures with this [here](http://matthewhutchinson.net) sometime soon.

Thanks,

Matt