add db migration, part of #79
reply to comment now nests to max. level of 1, part of #79
increase db version after migration
set parent to null if parent is not top-level comment
auto-correct wrong/invalid parent to max nesting level of 1
rm forward (reference) arrow
Merge branch 'feature/79', closes #79
add Python 3.4 to Tox and Travis CI
... but Travis does not ship Python 3.4