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
Should also test that `get_talks_by_room_and_time` doesn't return a presentation that occurred in the past, and the case where it doesn't find anything.

I also think the current test can be improved by inserting multiple presentations, otherwise this test would still pass if `get_talks_by_room_and_time` did no filtering and returned the first presentation it found.