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
add Python 3.4 to Tox and Travis CI
... but Travis does not ship Python 3.4
Bug in API: Reply count should also filter by the after value passed
Change in API: hidden_replies field instead of passed_replies
Client side changes for new API

* Handle hierarchal comments data format
* New comments fetching parameters: limit, parent, after
* Link to load more comments