Feature/inline images issue#2

In response to issue #2 and specifically [this challenge](https://github.com/yunojuno/django-trello-webhooks/issues/2), this adds support for image attachments from Trello to be rendered inline in HipChat, as <img> elements.

This implementation requires attached images to have one of several common file extensions in order to work; switching out the detection method is discussed within the implementation at /trello_webhooks.py@330 and would be relatively easy to amend.