Add Cache Buster to url

[vhalbwachs]

This fixes issues with browsers caching the tracking pixel if multiple requests go out the result in the same url being generated.

For example, if the following code were to run:

for (var i = 0; i < 10; i++) {
    _LTracker.push('clicked on search');
}

In most browsers, only one request would be made for the image, and the subsequent 9 requests would be returned from the browser cache.

This adds a CACHEBUST parameter with a random string of numbers to the url so that each request is unique. The additional parameter gets ignored by Loggly's collectors, so it doesn't modify the tracking payload.

[mostlyjason]

Deprecated by #25