Add Exercise: Complex Plane Operations

Summary:
Addition hints

Addition and subtraction fully working

Multiplication

Revert "Multiplication"

This reverts commit ddf5bfb1d2b893053049a6c7d6be02099fde8d02.

Typos

Removed errant file from different branch

Merge branch 'complex-plane-operations' of git://github.com/davemajor/khan-exercises into 39367-complex-plane-operations

Make Complex Plane Operations more like Adding Vectors.

Test Plan:
locally, Chrome/OS X
both problem types, seeds: 1, 3, 5, 7
made sure it:
- didn't blow up
- accepted the correct answer
- displayed expected hints

Reviewers: eater, stephanie

Reviewed By: eater

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D2032