implement payment logic

[espruyt]

Now comes the most complex part (I think), the payment logic to be used is the following:

• a % of the amount goes to an e-mail address. the webmaster can designate both % and e-mail address in the admin environment.
• the remaining % will be allocated as follows:
  - if task has url -> mobbr checks url if url has mobbr support (mobbr finds a script with a payment distribution key e.g. as on github, stackoverflow) -> the payment logic of that url is used (NB: for the remaining %, so applied to the amount minus the % taken by the webmaster/admin)
  - else there should be a form field asking the payer to enter the username or e-mail of the recipient(s) and behind that a box with a percentage (total of all recipients' shares should be 100%, if not use relative shares)