[IMP] website_sale: change 'pay now' button behavior.

Now: intercept the call, make an asynchronous call to create the tx, then continue to the acquirer. Previous this was done by
a redirection, leading to a get request beign done. However it should be a post request.
[IMP] payment addons: fixed submit buttons: now have type=submit (and not image or nothing) and removed name, because this may cause issues when trying to submit a form manually with type=submit and a name. Also improved js code in website_sale to correctly handle the click binding, the asynchronous call to the server to create the transaction, and then submit the form. As form submission is synchronous in javascript, we cannot use the deferred. Instead we prevent the click event from bubbling, wait for the server to create the transaction, adn then manually submit the form.
Merge pull request #224 from odoo-dev/master-payment-post-tde

[IMP] website_sale: correctly submit the payment form using a post request, correctly create the tx asynchronously