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.

Keeps the "windows", "windowsxp", "windowsvista" styles.

Reduces the size of a static bitcoin-qt.exe by ~400 kB.

It could just as well be on either part of the gitian build, but to safely put it on the Qt side would require bumping the filename, and every gitian user rebuilding it.
v0.5.3.1 put it on the Bitcoin side, and this is easier to work with and keep safe, so I'm moving it.

Use `qmake MINGW_THREAD_BUGFIX=0` to disable