Added support for aiohttp.__version__

Just to support being able to programmatically determine the aiohttp version using the standard convention of `package.__version__`

Just pulled the code from other setup.py implementations I have lying around that look awfully similar to what's at https://github.com/pypa/sampleproject/blob/master/setup.py linked from http://packaging.python.org/en/latest/tutorial.html#creating-your-own-project though admittedly mine is simpler (but works in most cases including this one). :smile: