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/primitives/transaction.h

@@ -134,7 +134,7 @@ public:
 
     uint256 GetHash() const;
 
-    bool IsDust(CFeeRate minRelayTxFee) const
+    CAmount GetDustThreshold(const CFeeRate &minRelayTxFee) const
     {
         // "Dust" is defined in terms of CTransaction::minRelayTxFee,
         // which has units satoshis-per-kilobyte.
@@ -145,7 +145,12 @@ public:
         // so dust is a txout less than 546 satoshis 
         // with default minRelayTxFee.
         size_t nSize = GetSerializeSize(SER_DISK,0)+148u;
-        return (nValue < 3*minRelayTxFee.GetFee(nSize));
+        return 3*minRelayTxFee.GetFee(nSize);
+    }
+
+    bool IsDust(const CFeeRate &minRelayTxFee) const
+    {
+        return (nValue < GetDustThreshold(minRelayTxFee));
     }
 
     friend bool operator==(const CTxOut& a, const CTxOut& b)