Transaction amount field fix

[krisboit]

GenericTransaction doesn't have an amount field, but specific classes from Zilliqa and Ethereum add an amount field with different names (amount and value).

export class ZilliqaTransaction extends GenericTransaction<IZilliqaTransactionOptions> {
    public version: number = 0;
    public pubKey: string;
    public code: Buffer;
    public amount: number;
...

and

export class EthereumTransaction extends GenericTransaction<IEthereumTransactionOptions> {
    public value: number;
    public chainId: number;
    public gasPrice: number;
    public gasLimit: number;
...