Update code to run under Python3 + Conda packaging

• Updated to work with Python 3
• Added unit tests to ensure basic functionalities of refscrub
• Updated list of platform specific dependencies to be installed when installing refscrub in dev mode (see setup.py)
• Installed and tested refscrub against Python 3.[6,7,8] using tox
• Formatted Python code to conform to the PEP8 style guide
• Created conda package for refscrub that can run on Python 3.[6,7,8]
• Updated .gitlab-ci.yml to run a linter and unit tests for Python 3.[6,7,8] in GitLab CI pipeline