initialize Api.__auth fixes #119

This eliminates `AttributeError: 'Api' object has no attribute '_Api__auth'` errors by initializing `Api.__auth` to None. Now, setting the authentication parameters incorrectly results in the proper error message: `TwitterError: {'message': 'Api instance must first be given user credentials.'}`.

Author: rbpasker

twitter/api.py

@@ -166,6 +166,7 @@ def __init__(self,
         self._debugHTTP = debugHTTP
         self._shortlink_size = 19
         self._timeout = timeout
+        self.__auth = None
 
         self._InitializeRequestHeaders(request_headers)
         self._InitializeUserAgent()