adding other_api_key to the API in general

I propose this change in order to support different api_keys if needed. Not 100% sure about the name, though.
Multiaccount platforms like monomi.co need this to use it!
Added required tests

HEADERS['Authorization'] = 'Token token="%s"' % (api_key if other_api_key is None else other_api_key)
makes sure that the other_api_key is always used if not None... A simpler way could be written like:
HEADERS['Authorization'] = 'Token token="%s"' % (other_api_key or api_key)
However, then the api would user the global api_key even if you send a blank "other_api_key"