Flesh out the MANIFEST.in file and add the 'license' field to setup.py

- add license field to setup.py
- include tests, examples, luigi/templates/*.html, and LICENSE

It's nice to have the tests and examples in the source archive, and the LICENSE file should be there too.
The setup.py file should also state the license.

I think I got these right.

You can always check to see what using the setup.py file includes with:

python setup.py sdist

which will (usually!) create a tarball which you can examine.