Add the ajax_comments module

There is an ajax_comments module that makes the comment add / edit form use ajax, which really makes for a better user experience.

theo implemented it in https://code.drupalcommerce.org/#/c/227/ and I tested it (using both alpha1 and latest -dev).
I found that the alpha status of the module really shows, since I could see multiple bugs (mostly centering around Preview not working).

So we decided to postpone this until the module becomes more mature, or we've had time to patch it ourselves (our plate is full right now).
This means post-Munich.