Feature/390 purchase with shortcodes

@mturalenka Please review.
WAT?
As the 'js_' prefix indicates, . lp_js_do-purchase has mousedown / click events bound to it, so when I click such a button, I automatically get a purchase dialog. How can that work, if the target is a regular page and I have to go to that page to purchase it?
On the other hand, .lp_purchase-link-without-function was specifically introduced to be able to style the button the same way as a 'purchase button', but not trigger a purchase dialog.