Test under all supported python versions and environments

[johann-petrak]

To test using different python versions, do tests while running from the command line and change the python environments.

• write a file with current (base) specs: conda list --explicit > spec-file.txt
• or create a spec file that contains only what is needed
• create an environment for a new python version, using that spec conda create -n myenv python=3.4 --file spec-file
• change to the environment: source activate python34
• when done deactivate: source deactivate