Adding and subtracting below zero

Before adding and subtracting negative numbers, users should practice on additions and subtractions starting and/or ending below zero.

I have copied "Adding and subtracting negative numbers" and changed it slightly (f.e. removed the negative numbers from the second term).

![kh.png](https://f.cloud.github.com/assets/2990187/1232/ea650ee6-40c3-11e2-8bd4-04465081ae72.png)
![kh2.png](https://f.cloud.github.com/assets/2990187/1244/58ec5d56-40c4-11e2-9562-4cff3a249952.png)
![kh3.png](https://f.cloud.github.com/assets/2990187/1254/fee7ebda-40c4-11e2-9018-ec12517ad9a0.png)