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
Do you want to try rewriting this method since this PR is doing lots of refactoring? It's poorly implemented. If we're checking if a presentation exists, we should check all fields, not just title and speaker. The query can also be a lot better, instead of a select all.