Add DeviceMessage property to BeaconEvent

[okdtsk]

Support device message from LINE beacon.
spec: https://devdocs.line.me/en/#webhook-event-object

[okdtsk]

As of 2017/4/26, flake8-docstrings has dependency issue related pydocstyle.
So test py35-flake8-src will be failed but #39 will fix this problem.

[be-hase]

How about using device_message instead of dm.
Other SDK adopt device_message(or deviceMessage)

[okdtsk]

Thanks for your review 🌱 I agree with your suggestion, I'll change that property's name.

[okdtsk]

I add another test when there is no 'dm' key inside webhook's json.

[be-hase]

LGTM 👍

[okdtsk]

@be-hase Thanks! I rebased this feature branch onto master (for #39) and squashed into one commit.

[okdtsk]

I found a failed test py35-flake8-other due to E501 - line too long at examples/flask-kitchensink/app.py:253:100, so fixed it.

diff --git c/examples/flask-kitchensink/app.py w/examples/flask-kitchensink/app.py
index b104be5..f6f2d7f 100644
--- c/examples/flask-kitchensink/app.py
+++ w/examples/flask-kitchensink/app.py
@@ -250,7 +250,8 @@ def handle_postback(event):
 def handle_beacon(event):
     line_bot_api.reply_message(
         event.reply_token,
-        TextSendMessage(text='Got beacon event. hwid={}, device_message={}'.format(event.beacon.hwid, event.beacon.device_message)))
+        TextSendMessage(text='Got beacon event. hwid={}, device_message={}'
+                             .format(event.beacon.hwid, event.beacon.device_message)))


 if __name__ == "__main__":