Implement v4 API functions for Withdrawal

[CosmicDuck]

Hello, nearly same code as for showDeposits which looks like is working for me.

` showWithdrawals()

def showWithdrawals(self, currency, **args):
    """Query on Withdrawals."""
    params = args
    avail_params = [ 'withdrawal_id', 'page' ]
    uri = f'{self.apibase}{currency}/withdrawals'
    p = ParameterBuilder(avail_params, params, uri)
    return self.APIConnect('GET', p)

`

[peshay]

Can you create a PR for it, with a unit test and documentation about that function in the README?