/**
 * Convert a string, number, or object into an array.
 * Especially useful for objects such as those that
 * come from simplexml_load_file(), etc.
 *
 * @param mixed $non_array
 *   Any string, number, or object.
 *
 * @return
 *   An "arrayified" version of $non_array. At minimum,
 *   this should always return an empty array.
 */
function arrayify($non_array) {
  if (empty($non_array) && $non_array !== 0) {
    return array();
  }
  return unserialize(serialize(json_decode(json_encode((array) $non_array), 1)));
}
Then use it like this to extract the JSON data:
$data = arrayify($bitcoin->listreceivedbyaccount());
print_r($data);