Adding a simple Python-as-calculator exercise

This just adds a simple math exercise that should be easy for students and get them used to work in the notebook. [View here.](http://nbviewer.ipython.org/urls/raw.github.com/jiffyclub/boot-camps/python-calculator/python/python-as-calculator/A%20Math%20Trick.ipynb) (nbviewer doesn't seem to show the empty code cells that should appear between the lines of text.)