Subtract fee from amount

Fixes #2724 and #1570.

Adds the
automatically-subtract-the-fee-from-the-amount-and-send-whats-left
feature to the GUI and RPC (sendtoaddress,sendmany).

src/qt/sendcoinsentry.h

@@ -51,6 +51,7 @@ public slots:
 signals:
     void removeEntry(SendCoinsEntry *entry);
     void payAmountChanged();
+    void subtractFeeFromAmountChanged();
 
 private slots:
     void deleteClicked();