Adds ability for sharing to twitter for submitted exercise

Adds ability for user to share with twitter when they submit an exercise.

"I just submitted the [submission language] [submission slug] at exercism.io. Leave me a nitpick at http://exercism.io/submissions/[submission.key]"

I created a presenter to do this, but would be more than willing to change how it's done if this isn't idiomatic, I'm fairly new to rails/big sinatra style applications.