Add expression 'acceptable format' for x^(y/z)

Summary:
Not the perseus style guide way, but nicer looking

{F13160}

Test Plan:
 * make an expression answer
 * look at acceptable answers

http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%22%2C%22widgets%22%3A%7B%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22expression%22%2C%22options%22%3A%7B%22value%22%3A%22x%22%2C%22form%22%3Afalse%2C%22simplify%22%3Afalse%2C%22times%22%3Afalse%2C%22functions%22%3A%5B%22f%22%2C%22g%22%2C%22h%22%5D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%7B%22content%22%3A%22Here's%20one%20way%20to%20make%20an%20angle%20with%20these%20points%2C%20but%20this%20angle%20is%20acute%2C%20not%20obtuse!%20%20%5Cn!%5B%5D(https%3A%2F%2Fka-perseus-images.s3.amazonaws.com%2F93fd1b71b55ca18f4a81ad0520df8c96e07125f4.png)%22%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22Here%20is%20the%20way%20to%20make%20an%20obtuse%20angle%20in%20this%20situation.%20It's%20an%20obtuse%20angle%20because%20its%20measure%20is%20greater%20than%20%2490%24%20degrees.%20%20%5Cn!%5B%5D(https%3A%2F%2Fka-perseus-images.s3.amazonaws.com%2F303cfd47cb758adc31af8f80d440827199c6bc63.png)%22%2C%22widgets%22%3A%7B%7D%7D%5D%7D

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6760