Minor py3k fixes

This PR implements:

- Fixes some minor py3k incompatibilities like: wrapping `print` statements, adding `from __future__ import absolute_import
` and etc.  (The only one worth looking at is changing `.iterkeys()` to `.keys()` since that might cause an performance impact.)
- Fixed some typos in comments and docstrings.
- Fixed test directory bug when running `python setup.py test`.
- Changed `.travis.yaml` to test python2 and python3.
- Renamed all files in `notebook_examples` from `name_test` to `name_examples` to avoid confusing when running `py.test`.