EtherChaintrade.amount is too big to handle in a standard way

[MiguelPeralvo]

The amout field in the EtherChaintrade class needs special handling: It doesn't fit in a long (it produces an overflow in some instances) and BigDecimal losses precision. BigInt conversion fails in some instances. We're using a String at the moment, to preserve the original information, but we'll have to refactor it once we complete the scope/initial analysis stage.