Sidescroller change: This is a first pass at emitting events for next and previous slide changes. With this change, events for overthrow-next and overthrow-prev will be triggered on the overthrow sidescroller whenever slides are advanced to the right or left, respectively. The event object of these events contains an overthrow object with properties such as active (which is an array of currently visible slides), and originalEvent (which is the event object that triggered the change - scroll, keydown, click, resize). Lastly, the sidescrollerElement.getActiveSlides() method is exposed on each sidescroller overthrow  and returns the active slides at any time when called. This addresses #42