added 'last_response' attribute to WebAPIMethod

the attribute is a instance of `requests.Response` and it will
hold a reference to the last response for the particular method.
This could be useful for debuging, or reading the body, which may
hold reason information, when http status code is not 200.

steam/webapi.py

@@ -42,6 +42,7 @@ class WebAPI(object):
         populates the name space under the instance
         """
         result = self._api_request(
+            None,
             "GET",
             "ISteamWebAPIUtil/GetSupportedAPIList/v1/",
             params={'format': 'json'},
@@ -79,7 +80,7 @@ class WebAPI(object):
     def _url_base(self):
         return "%s://api.steampowered.com/" % ('https' if self.https else 'http')
 
-    def _api_request(self, method, path, **kwargs):
+    def _api_request(self, caller, method, path, **kwargs):
         if method not in ('GET', 'POST'):
             raise NotImplemented("HTTP method: %s" % repr(self.method))
         if 'params' not in kwargs:
@@ -104,6 +105,9 @@ class WebAPI(object):
         f = getattr(requests, method.lower())
         resp = f(self._url_base + path, stream=True, **kwargs)
 
+        if caller is not None:
+            caller.last_response = resp
+
         if not resp.ok:
             raise requests.exceptions.HTTPError("%s %s" % (resp.status_code, resp.reason))
 
@@ -175,6 +179,7 @@ class WebAPIMethod(object):
     """
 
     def __init__(self, method_dict, parent=None):
+        self.last_response = None
         self._parent = parent
         self._dict = method_dict
 
@@ -240,6 +245,7 @@ class WebAPIMethod(object):
 
         # make the request
         return self._api_request(
+            self,
             self.method,
             "%s/%s/v%s/" % (self._parent.name, self.name, self.version),
             params=params,