Update [HttpApiClient](https://github.com/tailucas/investec-api-python/blob/master/investec_api_python/http_api_client.py) to support Investec Open API [3-Legged OAuth](https://developer.investec.com/za/api-products/documentation/SA_Open_API_-_Authorization).
Update HttpApiClient to support Investec Open API 3-Legged OAuth.