Remove envlist from tox.ini

I prefer executing 'tox' and having local results for my default version
of python.  Specifying a particular python version is easy with the '-e'
flag.
Add positional arguments to tox.ini

This allows you to pass additional flags to pytest.
Add standalone runtests.py script for running tests

This allows a user to not install py.test and/or tox in order to test
changes to the source code.
Cleanup imports and move __version__ to __init__.py
Add license field back to setup keywords
Remove makefile, not necessary anymore

Either tox or runtests.py offer better solutions.
Update travis configuration to use tox