Adding and subtracting rational expressions 6 (new branch)

Trying this again.

There's an issue with the expression editor - sometimes I can't work out what answer the editor will accept as correct. Would be nice to not use data-simplify, but if it's not included then typing the question is accepted as a correct answer.

Based on: https://www.khanacademy.org/math/algebra/rational-expressions/rational_expressions/v/adding-and-subtracting-rational-expressions

![adding and subtracting rational expressions 6](https://f.cloud.github.com/assets/219302/916289/43a9228a-fe88-11e2-9a78-cffdb51a8867.png)

Requires quite a lot of computation to solve.