Clean up code, fix link URL, and add badge to tweets

Did a couple of things in this PR:

* Tweets now include the story type in front of the title, if present.
* Links are now formatted to remove the `api-` part of the link.
* Included `StoryFormatter` as a module in Story instead of being standalone.
* No longer need to instantiate `Stories`.
* Moved Twitter client into module and out of global variable.

All the changes have been manually tested against a test twitter account. Cheers!