Fixed relative imports by making them explicit; removed sys.path alterations

So the reason we were getting trouble with the relative imports, where we import from the current directory, is because the syntax for relative imports changed with Python 3. They're still available, but they're differentiated from absolute imports with the 'dot' syntax for referencing the current directory, e.g.:

    from .app import app, db

After banging my head against a wall for quite awhile with this, I realized I just didn't know something about Python 3. We have to be more careful to actually use Python 3 if we're committed to doing so.

I went through and changed all our relative imports to this new syntax. I also cleaned up the imports in manage.py, so we're only importing what we use.