Adding functionality to get_balances to return for a specified account id only

[YinYin-blip]

Instead of the response having all accounts in a list. It would return a single account.
Example call:
print(luno.get_balances(account_id='12345678910'))

Example output:
{'account_id': '12345678910', 'asset': 'XBT', 'balance': '0.00', 'reserved': '0.00', 'unconfirmed': '0.00'}

[amustapha]

honestly, It's more efficient to fetch all balances and use a filter/find to return a specific balance.

[YinYin-blip]

@aamustapha I think we are saying the same thing. I just created this issue to start a conversation about whether this filtering would be useful to have in this client or if the filtering should be done by some calling function.

Here is my suggestion:

    def get_balances(self, assets=None, account_id=None):
        """
        :param account_id: filter api response to return a specific account only 
        """
        req = {
            'assets': assets,
        }
        if account_id:
            response = self.do('GET', '/api/1/balance', req=req, auth=True)
            for account in response["balances"]:
                if account_id == account["account_id"]:
                    return account
        else:
            return self.do('GET', '/api/1/balance', req=req, auth=True)

In terms of coding design, it might be a bad idea to have this client do the filtering AND the api calls. But we could also add a "filter" argument to some functions.

def get_balances(self, assets=None, filters=None):
luno.get_balances(assets=None, filters={account_id: "123456762"})