fix "send coins" via context menu in address book

- 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()

src/qt/walletmodel.cpp

@@ -56,6 +56,8 @@ int WalletModel::getNumTransactions() const
     int numTransactions = 0;
     {
         LOCK(wallet->cs_wallet);
+        // the size of mapWallet contains the number of unique transaction IDs
+        // (e.g. payments to yourself generate 2 transactions, but both share the same transaction ID)
         numTransactions = wallet->mapWallet.size();
     }
     return numTransactions;