Specify package_data in setup.py (minecraft.yaml must be included).

pymclevel requires `minecraft.yaml` to be shipped, otherwise we obtain
```
IOError: [Errno 2] No such file or directory: 'pymclevel/minecraft.yaml'
```
This is currently happening if installing pymclevel right from git via pip (i.e. via `$ pip install git+git://github.com/mcedit/pymclevel.git`). By default, only Python files are included in the distribution build. There is a canonical mechanism for including non-Python files into the distribution build, as explained here: https://docs.python.org/2/distutils/setupscript.html#installing-package-data

I have used the `package_data` method in order to include `minecraft.yaml` and the license as well as readme into the dist build. This is currently untested, but this method generally works for my packages.