app - refactor to allow Tornado settings to be injected at runtime

This is moving towards using dependency injection to allow testing
of a persistent database. However this commit does not change any
functionality.
tox - include tests in coverage tests, delete old coverage stats on run
tests - fix import of local module for Python 3

After running coverage stats on tests it turned out that the imports
didn't work properly on Python 3.
gitignore - ignore coverage files from parallel-mode