Adding Version Attribute to QuantEcon class

This PR adds a version attribute to the QuantEcon class that is derived from ``setup.py``. When you run ``python setup.py install`` this generates a ``version.py`` file and places it in the ``quantecon`` package for use by ``__init__.py`` which attaches the ``__version__`` attribute to the package. 

Version can be checked using: 

```python
import quantecon as qe
qe.__version__
```

Future Work
---------------
  1. Pandas generates the ``version.py`` file on build and therefore places a ``version.py`` file in the ``site-packages`` location without adding it to the repository. For now I have added a docstring to the file with a reminder not to edit it - as it will get overwritten with every install command.