Fix ZinniaCalendar timezone usage.

Entries calendar was being built using UTC dates. However, fetching entries from archive seem to use local time. This caused strange behaviour when calendar renders particular day with a link (meaning there were entries posted on that day) but after following the link zinnia shows no entries (for small timezone shifts such behaviour is easily reproduced for posts near the midnight).

Proposed fix just makes ZCalendar timezone-aware when generating its days_entries.