Add support for a Payment Protocol to Bitcoin-Qt.
Payment messages are protocol-buffer encoded and communicated over
http(s), so this adds a dependency on the Google protocol buffer
library, and requires Qt with OpenSSL support.
Switch to using Qt's QLocalServer/QLocalSocket to handle bitcoin
payment links (bitcoin:... URIs)
Reason for switch: the boost::interprocess mechanism seemed flaky,
and doesn't mesh as well with "The Qt Way"
qtipcserver.cpp/h is replaced by paymentserver.cpp/h
Click-to-pay now also works on OSX, with a custom Info.plist
that registers Bitcoin-Qt as a handler for bitcoin: URLs and
an event listener on the main QApplication that handles
QFileOpenEvents (Qt translates 'url clicked' AppleEvents into
QFileOpenEvents automagically).
