Make $post_id protected, not private

[benlk]

Changes

Makes the property $post_id of Largo_Byline protected, not private, so that when Largo_CoAuthors_Byline extends Largo_Byline and uses Largo_Byline's methods __construct and populate_vars, Largo_CoAuthors_Byline will be able to read the variable that it tries to set.

Otherwise, anything in Largo_CoAuthors_Byline that uses $this->post_id gets a big fat null, and usually tries to work based off of the current post in The Loop.

This is only a bug when calling largo_byline with a post ID specified.