Exclude tests and include sql files in cnx-authoring package

Remove include_package_data=True as it looks for MANIFEST.in instead of
using the package_data that we specified in setup.py.

Tested by running:
```bash
rm -rf cnx_authoring.egg-info dist
python setup.py sdist
```

Close #51
Merge pull request #63 from Connexions/package-data

Exclude tests and include sql files in cnx-authoring package