Site depends on Python 2 (doesn't work with Python 3)

This is because of the use of [pygments.rb](https://github.com/tmm1/pygments.rb) for syntax highlighting. As far as I can tell, pygments requires Python 2.x only. 

Under Python 3, running `jekyll serve` yields::

```
  Liquid Exception: Failed to get header. in drug/event/reference/index.md
jekyll 2.0.3 | Error:  Failed to get header.
```

Switching to Python 2 fixes it. 

This also adds a `.python-version` file for anyone using [pyenv](https://github.com/yyuu/pyenv) like me. It doesn't affect anyone not using that system.