Improved card creation

Hi @meinside,

I've tinkered a little with card creation, and this is the result. Many lines have been touched in the process, because I moved the plugin's classes into their own module namespace, which I thought was preferable compared to having them in the main namespace. Hopefully this is okay with you.

The main change I've done is to refactor card formatting into the `CardFormatter` class. Cards now include the URL to their issue, and the due date is ommited if none has been set in the ticket.

Let me know what you think, and if you're willing to pull this in :smile: