Payment To Membership Integration Fix

From previous job - when user pays via Braintree, e-mail is sent with link to 'register'. User should be able to register and be automatically set up as

- s2member level 2
- bbpress user level 'participant'
- in bbpress name set to 'first name'

To be done only if the basic integration ability is present. If it's a big custom job, let's not worry about this one for now - if it's more of a bug situation or available with s2member integration points, then let's do it.