Flesh out the readme. Add a sample Python script.

Explain what NailGun is, why it exists, its scope and limitations, and provide
other miscellaneous information. Add a script named `get_org_envs.py` that
clearly shows the need for an API abstraction. The script completely satisfies
both the Python 2 and 3 versions of flake8 and pylint.
Drop unnecessary lines from `/.gitignore`

An excessively large `.gitignore` file makes it easy to miss subtle issues. The
`.gitignore` file will grow as time goes on, but in a precise manner and on an
as-needed basis.