Make code compatible with Python 2 and 3

Fix #10
Use pylint error names, not error codes

Pylint 0.25.3 and above allow one to reference errors using symbolic names
instead of error codes. Names are more human-friendly than error codes, so use
them. See:
http://docs.pylint.org/faq.html#do-i-have-to-remember-all-these-numbers
Fix docs, be strict when building documentation

Fix an error in the API documentation. Add the `-W` flag to `SPHINXOPTS` in
`docs/Makefile`. This option makes sphinx "turn warnings into errors". The
purpose of this change is to make Travis fail if any commit breaks the
documentation.
Add "Examples" page to documentation, show TOC

Move `get_org_envs.py` to the `docs` directory and include it in the "Examples"
page. Show the table of contents on the landing page.

Fix #18.
Release version 0.1.0

Changes in this release:

* Make code compatible with Python 3. (issue #10)
* Move `get_org_envs.py` to the `docs` directory. (issue #18)
* Fix documentation errors. Be more strict when building documentation, thus
  preventing future errors from creeping in.
* Make most pylint directives human-readable.