Added authentication parameter for base_url

Authentication parameter for base_url, default endpoint 'https://secure.eloqua.com/' is no longer valid for new users.

See: http://topliners.eloqua.com/community/code_it/blog/2012/11/30/using-the-eloqua-api--determining-endpoint-urls-logineloquacom
Note: This change should mirror the authentication at https://github.com/fredsakr/eloqua-php-request