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 ?
Merge pull request #2679 from vhf/patch-1

Too many bitcoins allowed as amount. (Issue #2401)