Simplifying Square Roots of Negative Numbers

This branch has an implementation of the exercise Simplifying Square Roots of Negative Numbers as discussed in the Trello card https://trello.com/c/ohMXloTH. List of significant changes include:

1. Added a new exercise called "simplifying_square_roots_of_negatives.html"
2. Added unit tests for the new exercise.
3. Fixed a bug in answer-types.js where solution was being set to "undefined" for the Multiple answer type. Fixing the bug was necessary in order to write the unit tests.

This is my first contribution so any feedback would be appreciated.