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
Typically Python devs don't care about public/private methods. However, the generally accepted way to make a "private" function is to prefix it with ONE underscore. TWO underscores does some name mangling (as you've seen) and is generally discouraged. I understand that this style is used throughout Freeseer, but going forward it shouldn't.