Added exercise for converting percents to fractions and mixed numbers.

Here is an exercise unit for converting percents to fractions and mixed numbers, in response to Trello card #389 (https://trello.com/c/jnAsLuk2).

Edit: Sandcastle URL: http://sandcastle.khanacademy.org/media/castles/ynagasaki:ynag-convert_percents_fractions_mixed/exercises/converting_percents_to_fractions_and_mixed_numbers.html