Better python 3 support

[AlmogCohen]

• Add travis, coverage, pypi and python versions badges to README.
• Add Pipfile for development.
• Add python 3.7 to tox and travis.
• Update python3, CPython and PyPy support in setup.py keywords.

To enable the test coverage badge, please go to https://coveralls.io/ and just list this repo. Same as you did for the travis builds.

[AlmogCohen]

I would also recommend releasing version 0.4.1 to PyPi if you accept this PR so this package would be marked as python3 compliant on PyPi.

If you want to also set up automatic CI/CD (which will deploy tagged versions to PyPi) you can take a look at the travis.yml file under the deploy section here: https://github.com/algosec/algosec-python/blob/master/.travis.yml . TravisCI has good documentation about it. I really liked it and use it together with the bumpversion tool to quickly deploy new versions on demand.

[AlmogCohen]

Anyone?

[benkonrath]

Seems like a good addition. @armooo Do think you'll have some time to review / merge this and make a new release?