Support expired/known methods looking at day (timezoned) of next review date only

[rgravina]

Normally, the next review date is calculated as some number of days after the review timestamp.

It's convenient for users to review words at the same time every day (morning, night etc.). To do this, all words expiring on e.g. 2012-12-01 should appear in the expired list at exactly 2012-12-01 00:00.

This also implies we know the timezone of the user, as midnight differs based on the timezone the user is in.

The User model should supply the users timezone. A setting for this should also exist in Chuhi (demo app using this gem).