Too many bitcoins allowed in amount. (#2401)

Using magic number doesn't seem right. Could we factor this out, together with https://github.com/bitcoin/bitcoin/blob/master/src/bitcoinrpc.cpp#L96 ?
And what about BitcoinUnits::parse() as well ?

src/qt/bitcoinunits.h

@@ -38,6 +38,8 @@ public:
     static QString description(int unit);
     //! Number of Satoshis (1e-8) per unit
     static qint64 factor(int unit);
+    //! Max amount per unit
+    static qint64 maxAmount(int unit);
     //! Number of amount digits (to represent max number of coins)
     static int amountDigits(int unit);
     //! Number of decimals left