Adding and subtraction rational expressions 0.5

Adding and subtracting two expressions with a common denominators and each of the form Ax / B or A / Bx.

Requires the RationalExpression object from #44676

![add_sub_rat_expr_0 5](https://f.cloud.github.com/assets/219302/394029/a0b72c3a-a7ab-11e2-84e6-28f78ab76f6c.png)