Square wave generator example code added

It generates 7.5 10 12 16 Hz square waves on pin 21 26 12 6
simultaneously
Here are the waves on the oscilloscope
![t0001](https://cloud.githubusercontent.com/assets/8993036/5157336/030365be-7340-11e4-9864-d34de8fe96f1.png)
![tek00001 2](https://cloud.githubusercontent.com/assets/8993036/5157338/032081ee-7340-11e4-95fd-28d7f174f7e3.PNG)
![tek00003](https://cloud.githubusercontent.com/assets/8993036/5157337/030469dc-7340-11e4-86da-ae5d133b2dea.png)