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
To be honest, I'm not really sure what the proper timestamp is that Freeseer expects, I think it's changed a few times. I find it confusing that the schema uses `TIMESTAMP` for everything
```
Date timestamp,
StartTime timestamp,
EndTime timestamp,
```
and then parse the results rather than `DATE` and `TIME` types.

IIRC, it was to make the schema upgrade easier, which only contained the field `Time timestamp` to represent the date and time.

It might be worth looking into seeing if we can create a new schema version that uses the proper types. Though this will require some refactoring, such as the code snippet you posted above.