Enable smooth scrolling

This pull will close #1 and #4.

Added support for animated scrolling. Animation occurs both when `stepAhead()` and `stepBack()` functions are called, but also when an anchor link is clicked (uses new `scrollToAnchor` function). In both cases the
animation is controlled by a new helper function, `smoothStep`, which accepts a single parameter of new scroll value (an integer).

This solution uses a mixture of of jQuery and plain old JS to achieve the desired behavior.