Expose the 'jump' method publicly

Thanks for the great plugin!
I was using this on a project and I needed to use the carousel effect, but be able to change the current image occasionally, i.e. auto rotating unless the user has their mouse over the flipster container.
To do this I needed the 'jump' method available publicly.  I also noticed in [issue #8](https://github.com/drien/jquery-flipster/issues/8) there was mention of exposing some of the methods publicly, so I created this PR.

Example usage:

    $('#flipster').flipster();
    $('#flipster').flipster('jump', 'left');

Let me know if there's anything that needs changed to get this merged.
best,
-Joe