Google Analytics Extension to track Page Scroll Depth

# Purpose
This plugin setups GA event and timing tracking to report on user page scroll depth. 

## How it works
It determines page size and scroll position and then reports based on pre-determined percentages to GA once these points are met:

* 0% (Baseline)
* 25%
* 50%
* 75%
* 100%

Once a point has been met, it can be counted again during a single pageview.

## GA reporting
This utilizes event tracking to tie each event to a specific page. An analytics user can view any page and determine the number of events for each percentage point.
In addition, the [`_trackTiming`] (https://developers.google.com/analytics/devguides/collection/gajs/methods/gaJSApiUserTiming) API is being used to determine how long it takes for the end-user to hit each percentage point.