Fix API by adding 'responses_of' method

`responses_of` replaces `response_of`, since each request can have
several matching responses now.

This breaks backwards compatibility if you are using the
response_of method, so a version bump will be required.

vcr/cassette.py

@@ -61,10 +61,10 @@ class Cassette(object):
         self.data.append((request, response))
         self.dirty = True
 
-    def response_of(self, request):
+    def play_response(self, request):
         '''
-        Find the response corresponding to a request
-
+        Get the response corresponding to a request, but only if it
+        hasn't been played back before, and mark it as playe.d
         '''
         for index, (stored_request, response) in enumerate(self.data):
             if requests_match(request, stored_request, self._match_on):
@@ -75,6 +75,19 @@ class Cassette(object):
         # if the cassette doesn't contain the request asked for.
         raise KeyError
 
+    def responses_of(self, request):
+        '''
+        Find the responses corresponding to a request.
+	This function isn't actually used by VCR internally, but is
+	provided as an external API.
+        '''
+        responses = [resp for req, resp in self.data if requests_match(req, request, self._match_on)]
+        if responses:
+            return responses
+        # I decided that a KeyError is the best exception to raise
+        # if the cassette doesn't contain the request asked for.
+        raise KeyError
+
     def _as_dict(self):
         return {"requests": self.requests, "responses": self.responses}