add modules to PYTHONPATH before starting kernels

[minrk]

In the interest of making colaboratory a proper Python package,
it may make sense to move the utilities into the colaboratory/ directory,
which would make them importable without modifying the path.

closes #34

[KesterTong]

Thanks Min. We would prefer the add colabtools to the path, because our internal users are already using import colabtools.

[minrk]

Do you plan to put many things in /modules? This seems an odd behavior. If you put colabtools in the top-level dir, you wouldn't need to touch the path.

[KesterTong]

good point. We had originally thought of adding a modules directory to package extra modules with the colab installation. Given the current structure, that makes a lot less sense. So I'm fine with your idea of adding colabtools to the top level directory. Will do that instead of merging this PR.

[minrk]

Sounds good. Then later, colaboratory and colabtools can be proper installable Python packages.