Allow specifying timezone for today-at-midnight

Right now there's no easy way to specify midnight today in a given timezone through clj-time. The best that I can think of is to get today's date in that timezone using `(to-timezome (now) tz)` and construct date-midnight from the year-month-day components, which is fairly messy.

This patch exposes the timezone parameter in DateMidnight's constructor to make that much easier.