Feature/390 purchase with shortcodes

@mturalenka Please review.
Fixed comments and public to private visibility.

Because of 2 reasons:
- methods $this->assign and $this->get_text_view is not available from helper context. Where it might be moved. And "the_purchase_button" are too simple to divide it to helper and controller part.

- LaterPay_Controller_Post->the_purchase_button() called as callback and LaterPay_Controller_Shortcode->the_purchase_button called inside LaterPay_Controller_Post. So I suggest to not merge it.