1.6 release - CircleCI 2.0, tox, py35+ tests and support (#23)

* CircleCI 2.0 and tox

Decided to use tox, so that you can:
- run tests locally in multiple python versions
- have a consistent testing platform across CI and local environment

Also:
- updated readme
- refactored setup.py ... made it not a fatal exception if `python
setup.py` is run without an installed `numpy`

* fix flake8 issues

* blackified code

* fixed tests code to support py35+

* updated cpp library with the latest cython 0.29.14 (previously 0.23.4)

* 1.6 release