[5.0] Added the ability to change the `$pageName` property

Changed the `$pageName` property to be static. There was no way to change the `$pageName` to another value without having is static when using the Paginator with the query builder.

This allows the application to set another `$pageName`.

I am aware you with this change can't have separate `$pageName` per Paginator, but when does an application have that? I've never ever seen that.

With this change, you can change the `$pageName` to another value by using

    \Illuminate\Pagination\Paginator::$pageName = 'sida';

just like you would with the static method `currentPageResolver`

    \Illuminate\Pagination\Paginator::currentPageResolver(function() {
        return $this->app['request']->input('sida');
    });

_For those who don't know, `$pageName` sets `?pageName=N` in the views while `currentPageResolver` looks for `?pageName=N` in the url._

The ability to be able to do this is very important for websites not using English urls.