requests_oauthlib dependency name will only install from pip, requests-oauthlib works on both conda and pip

Python 3.4 is EOL and support for it has been removed by
the requests package that we depend on.

Python 3.3 is EOL, and is failing on Travis.

This optimizes setup.py for cases when pytest-runner is not needed,
using the approach that is suggested upstream:

https://pypi.python.org/pypi/pytest-runner#conditional-requirement

This reverts commit 1650e0e7.
Upstream restored handling of the adding_headers arg in 0.6.1.
Going back to using that name means we're still compatible with
any version of responses after 0.3.0, except 0.6.0. Also adjust
setup.py to express this correctly.

Remove python2.6 from
- Travis
- Tox
- PyPi tags
- documentation and readme

Also, Simplify an import clause.

This fixes #133

Looks like a buggy pip version could be the root cause, but
nevertheless, we shouldn't need to specify that we're depending on
requests explicitly when requests_oauthlib depends on it.

(https://stackoverflow.com/questions/27497470/setuptools-finds-wrong-package-during-install)

Add OAuth and update docs

- Add zip_safe flag

Prepare for Python 3.x support
- Use print function
- Use compability methods from six