#Description jQuery plugin that renders an ad only when 50% of it is visible in the viewport for a set time (default 1000ms).
#Usage $(selector).viewableImpression(options);
##options (with defaults): { interval: 100, offset: 0, callback: function(options){} }
###interval
Frequency to check scroll position on scroll. 100 will check at an interval of 100ms on scroll.
###offset
Offsets when to fire the callback function. For example, 500 will call the callback function 500px before the element is in view.
###callback
Callback function that is called when the element is in view. Takes one argument (the options passed in). this in this scope references the DOM element that is now in view.