Use nested where for where arrays

I think the way it is currently set up is counter intuitive. Take the following piece of code as an example:

	$attributes1 = ['fname' => 'Taylor', 'lname' => 'Otwell'];
	$attributes2 = ['fname' => 'Joseph', 'lname' => 'Silber'];

	$users = User::where($attributes1)->orWhere($attributes2)->get();

With the way it is currently set up, this'll run the following query:

	select *
	from `users`
	where `fname` = 'Taylor' and `lname` = 'Otwell'
	or `fname` = 'Joseph' or `lname` = 'Silber'

I think this is obviously wrong.

After the PR, the code above will run this query:

	select *
	from `users`
	where (`fname` = 'Taylor' and `lname` = 'Otwell')
	or (`fname` = 'Joseph' and `lname` = 'Silber')