adding option for animated gif capturing (OSX/Mac only via VideoSnap)

Using VideoSnap and ffmpeg (and 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) The longer
the time, the larger the animated gif file-size. Some extra points to note;

* Currently mac only
* If -a 0 (or not present) lolcommits will operate as normal
* Uses VideoSnap https://github.com/matthutchinson/videosnap

I've tested with a few of the plugins and it appears to work OK! Also updated
README.md with instructions on how to enable and configure and an example
mark as pre-release
add videosnap to rake release as executable
bump prerelease version
adjusting frame pick and delay
minor rewording on help info
bump pre
animated gif frame delay calculated from movie fps
adjust version and changelog for v0.5 release!
Merge pull request #114 from mroth/v0.5_wip

v0.5 Release (animated gif lolcommits)