Payment Protocol Work

Add support for the Payment Protocol ( https://en.bitcoin.it/wiki/BIP_0070 ) 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 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

Test plan:
  https://github.com/gavinandresen/QA/blob/master/PaymentRequestTest.md

This adds a dependency to the Bitcoin-Qt build:  you need the protocol buffer compiler and library (see doc/readme-qt.rst)