Link -g option (group of pictures) to the video frame rate

[philipstarkey]

Youtube recommends a GOP of half the frame rate. Currently autoscrub only correctly applies this if your framerate is 30.

There was a simple fix done in changeset 38dde7e (which is not included in the current releases) however, we should really be able to determine this from the video input file automatically without having to expose it as a separate command line option