[5.0] Added fluent setter for `$pageName` for Paginator

Continuing on pull https://github.com/laravel/framework/pull/6384.

This allows you to change the `$pageName` property fluently.

    Model::paginate(20)->setPageName('sida');

Still not really fond of this, as you need to have

    \Illuminate\Pagination\Paginator::currentPageResolver(function() {
        return $this->app['request']->input('sida');
    });

somewhere else (in a new service provider etc). Would have been better if you could change it in only one place, but at least it's better then not being able to change it at all.
No trailing whitespace please.