Payment Protocol Work

Add support for the Payment Protocol ( https://gist.github.com/gavinandresen/4120476 ) to Bitcoin-Qt.

Web page where you can create test PaymentRequests, signed with either the 'bitcoincore.org' website SSL certificate or a 'gavinandresen@gmail.com' StartSSL free email certificate:
  https://bitcoincore.org/~gavin/createpaymentrequest.php

Source code for that website is available at:
  https://github.com/gavinandresen/paymentrequest/tree/master/php/demo_website

Loose ends:

1) QA test plan, and testing
2) Modify the Windows .nsi file to set the right registry entries for payment request MIME types
3) Write up BIPs for PaymentRequest/etc messages, document the bitcoin: URI extension, and describe the MIME types used for the messages.