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 don't understand this comment. If you're testing if you can insert an empty presentation without raising an error, I suggest creating a separate test for that.

I also don't understand why there's a speaker arguments, because looking at the `Presentation` class init method, the default value for that is an empty string.