Added slideTo, slidePrev and slideNext public methods

Hi gilbitron,

I've added support to public function as slideTo, slidePrev and slideNext.

The functions works using your native implementation, the only difference is that I made the internal function triggered by namespaced events (slideto.nivoslider, slideprev.nivoslider, slidenext.nivoslider).

Than I exposed those events triggering through public functions.

Feel free to pull or comment,

Regards,

Fabio