Get The Difference Between Two Dates

In the ``Date.php`` class define a method to get the difference of two dates in whatever format the user needed them in.

E.g.

````php
Date::differenceInDates($date1, $date2);
````
> returns in ms by default, but 3rd param could be supplied to change return format

````php
Date::differenceInDates($date1, $date2, 's');
````
> returns difference in seconds