Migrating to Python3

Hi Matt,

I have made following modifications to support Python3. Please take note that they are not gonna work on Python2. To support both of them, personally, I have no idea either to have only one set of code with "if-else" all over the place or making extra effort supporting two set of code.

- Modify PubChemPy to support Python3 (no backward compatibility with Python2).
- Unit testing passed successfully.
- Add IPython notebook based on unit testing.

I have converted console unit testing into IPython notebook. Following is the URL to view it on browser.

http://nbviewer.ipython.org/urls/raw.githubusercontent.com/ekaakurniawan/PubChemPy/master/PubChemPy.ipynb?create=1

Regards,
Eka A. Kurniawan