Adding and subtracting rational expressions 3

Adding or subtracting a fraction from and expression in the form (Ax + B) / (Cx + D)
![adding and subtracting rational expressions 3](https://f.cloud.github.com/assets/219302/820333/7086d45c-efc2-11e2-86ce-47e5f5392b3c.png)

I'll also accept factored answers, but I need the update to rationalexpressions.js from another branch first.