Reimplement click-to-pay links. Add OSX 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).