unit test and refactor database.py

Updates to ``tests/framework/test_database.py`` and ``framework/database.py``.

- Replaced ``test_database.py`` ``unittest`` references with ``pytest`` references
- Removed test methods that checked the return type of database functions
- Added test methods that checked the return values of database functions
- Modified database functions to handle python strings properly
- Added bound values to the add_presentation and update_presentation database functions.
- Removed dummy record inserted by default.

Related gh484
I went through the tests and broke them up (to prevent anyone from having to do it later and while I am still familiar with what the tests are supposed to do.)

I fixed the speaker arguement. I think I thought that the code in ``frontend/cli.py`` below would try to insert the empty speaker field. However, that is already done in the class init as you mentioned.
```
	if args.action == "add":
	    presentation = Presentation(args.title,
                                        speaker=args.speaker,
                                        room=args.room,
                                        event=args.event)
	    db.insert_presentation(presentation)
```