Fix #521 Modifying a talk using the details view hides the save button

- Enable Save Button only when an input field is modified by calling function
  enable_save
- Disable Save Button on init, and when input fields are autofilled or
  cleared
- Allow Save Button to be used for updating existing talks and adding new ones
  by calling function add_or_update_talk
- Edit docstring for function unsaved_details_exist
- Add docstrings for update_talk and create_presentation
Fix #537 Translations do not work for RecordTool and ConfigTool

Calling self.app.installTranslator seems to allow the translations to go
through to the 3 apps.

Signed-off-by: Thanh Ha <thanh.ha@alumni.carleton.ca>
Update translation files

Signed-off-by: Thanh Ha <thanh.ha@alumni.carleton.ca>
Update en_US translation file strings

Signed-off-by: Thanh Ha <thanh.ha@alumni.carleton.ca>
Update resources

Signed-off-by: Thanh Ha <thanh.ha@alumni.carleton.ca>
Switch to pytest as our test runner

http://pytest.org/ for more information. Old tests done in the
traditional unittest style will continue to function.