Add setup

Hi 

I have created the simple `setup.py` to improve installation process via packaging.
The coursera-dl can be submitted to PyPi and the installation for end user will be much simpler:
```pip install coursera-dl``` (all dependencies will be installed automatically)
and the usage will be - 'you can run command `coursera-dl ...`  from any directory and without worrying of PATH variable'

While it is not yet available on PyPi it can be built and installed locally.
to build a package:
```python setup.py build sdist```
to install it:
```pip install dist/coursera-dl-*.tar.gz``` 

P.S. The only thing which can be potentially broken is a Windows support.