Add clues for common mistakes/misconceptions

See this [sandcastle](http://sandcastle.kasandbox.org/media/castles/Merbs:clues/exercises/clues_example.html) and go through the three examples.

Provides a clue if the user's guess is a common mistake/misconception (arithmetic error, misunderstanding, wrong format). Gives the exercise-creator the ability to add `data-clues` property to solution in form `'{"guess":"message", "guess_2":"message_2"}'` with an option to pass an array as the value: `'{"guess":["message",
"count_as_empty"]}'`, where count_as_empty can be set to true to prevent the guess from being marked incorrect (a more lenient approach).

Only on the answer-type `expression`, for this commit. Just a proof of concept.