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/walletmodeltransaction.h

@@ -35,8 +35,10 @@ public:
     void newPossibleKeyChange(CWallet *wallet);
     CReserveKey *getPossibleKeyChange();
 
+    void reassignAmounts(int nChangePosRet); // needed for the subtract-fee-from-amount feature
+
 private:
-    const QList<SendCoinsRecipient> recipients;
+    QList<SendCoinsRecipient> recipients;
     CWalletTx *walletTransaction;
     CReserveKey *keyChange;
     CAmount fee;