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.
- the send coins context menu entry was not working anymore, because
a non current version of #2220 was merged onto current master
- also removes some unneeded spaces and adds a comment to
WalletModel::getNumTransactions()
