Added support for limited access keys (to be used with or in substitute of api_keys for NucleusClient auth)
Example usage:
c = nucleus.NucleusClient(limited_access_key="<LIMITED_ACCESS_KEY>")
c = nucleus.NucleusClient(api_key="<API_KEY>", limited_access_key="<LIMITED_ACCESS_KEY>")
c = nucleus.NucleusClient(api_key="<API_KEY>")