Add TestBackend for testing

Fixes #67.

django_slack/__init__.py

@@ -288,6 +288,23 @@
 
 This allows you to, for example, use the Requests library to whilst sending
 your message asynchronously via Celery.
+
+Testing
+-------
+
+You can set ``SLACK_BACKEND = django_slack.backends.TestBackend`` when testing
+to store messages to inspect them later::
+
+    from django_slack.utils import get_backend
+
+    backend = get_backend()
+    backend.reset_messages()
+
+    # Do something that triggers the sending of a message
+
+    messages = backend.retrieve_messages()
+    assert len(messages) == 1
+    assert messages[0]['text'] == expected_text
 """
 
 from .api import slack_message

django_slack/backends.py

@@ -71,5 +71,28 @@ def send(self, *args, **kwargs):
         self._send.delay(*args, **kwargs)
 
 
+class TestBackend(Backend):
+    '''This backend is for testing
+
+    Before a test, call `reset_messages`, and after a test, call
+    `retrieve_messages` for a list of all messages that have been sent during
+    the test.
+    '''
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.reset_messages()
+
+    def send(self, url, data, **kwargs):
+        self.messages.append(data)
+
+    def reset_messages(self):
+        self.messages = []
+
+    def retrieve_messages(self):
+        messages = self.messages
+        self.reset_messages()
+        return messages
+
+
 # For backwards-compatibility
 Urllib2Backend = UrllibBackend