Fetch current account balance

Now that your account is activated, you can check its current balance. This balance is updated every time a new bank statement is imported. This happens on a regular basis and depending on your setup, could happend multiple times a day. Initially it is ```null``` so there wont be any confusion
The response also includes an account's current configuration. Some of this settings can be changed later on, most are fix once the account has been submitted.

    curl -XGET "/accounts/DE02700205000007808005" \
      -H 'Accept: ' \
      -H "Authorization: Bearer "
      "name": "Fake Account",
      "iban": "DE02700205000007808005",
      "bic": "BFSWDE33MUE",
      "balance_date": null,
      "balance_in_cents": null,
      "creditor_identifier": null,
      "subscriber": "USER_ID",
      "url": "https://fakebank.url/ebics",
      "partner": "PARTNER_ID",
      "host": "HOST_ID",
      "status": "activated",
      "callback_url": null,
      "_links": {
        "self": "http://localhost:5000/accounts/DE02700205000007808005",
        "statements": "http://localhost:5000/statements?iban=DE02700205000007808005",
        "transactions": "http://localhost:5000/transactions?iban=DE02700205000007808005",
        "ini_letter": "http://localhost:5000/accounts/DE02700205000007808005/ini_letter"