Port to Python 3

[csparpa]

The portability assessment shall lead us to decide between one of the following actions:

• patch pyowm so that it is runnable under a Python 3.x env
• write a parallel pyowm releases branch specific for Python 3.x users

A good start is to run the 2to3.py utility

A more convenient way is to use tox

[csparpa]

Implemented with release 1.2.0: ported code to Python 3.3.5