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 am not 100% sure how to test this. I have run the unit tests with the change and nothing seems to break in the test report (from a quick grep it looked like the test_database module had been the only piece of software using them). The talk editor does not seem to throw any error messages when I add or remove talks. There are a lot of python errors that appear in my log when I click and unclick prepare to record, but this occurs in the master branch as well, so I do not think this is related. I can look for more database accesses