Fix py2/py3 compatibility due to relative import issues

[gschneider-r7]

Description

Use relative imports since this is a proper package. Get rid of the hacky scripts that make the project importable for the demo and test scripts, since they are no longer needed.

Motivation and Context

Fixes #23

How Has This Been Tested?

So far I have tested it locally with virtual environments for 2.7 and 3.6. I also have this branch running on travis-ci for the full suite of tested versions, which is passing after fixing imports on the tests.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [ ] I have updated the documentation accordingly (if changes are required).
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[gschneider-r7]

Tested with a built wheel that the relative imports work correctly when fully deployed on both Python 2.7 and 3.6.