Add support for python3

[rarguelloF]

Solves https://github.com/jhaals/ansible-vault/issues/65

I added some changes to the encoding/decoding to support python3 (made sure the tests still pass for python2.7).

I also made some extra changes:

• Added a Makefile with make build and make test commands for running tests locally.
• Updated the travis build to run flake8 and the tests both in py2 and py3
• Updated dockerfiles to use python-slim for lower image sizes.
• Updated to latest vault version in tests (also fixed a deprecation warning).
• Added a .editorconfig to the project (http://editorconfig.org/)

Please let me know if you don't like any of the changes so I can fix them.

[jhaals]

brilliant @rarguelloF 💯