Auto-Focus Editor and Run Code on Shift-Enter

This PR aims to allow users to work through the early 4clojure faster by:
- Running the problem when Shift-Enter is pressed.
- Auto-focussing the editor when the problem page is loaded.

Keyboard short-cuts are mentioned in issues #151 and #240. This change does not cover all the short-cuts mentioned in #151 and uses a different key combination than issue #240.

Let me know if a different key combination is required or if there is a straight-forward way to implement the other keyboard shortcuts mentioned in #151. These changes seemed like a quick win to me.