Add option to store the datepicker field as local time.

When using Fieldmanager_Datepicker, the time string is converted to a timestamp. The timestamp is stored for that time in GMT, therefore if I'm in EDT and I enter 6/25 2:00pm, I'm _really_ storing 10:00am.

Currently, FM uses `strtotime` to generate the date, which in WordPress guarantees GMT. The main question here is: **When someone enters a date, is it assumed to be in the site's timezone?** I would argue that yes, it always is. Changing this across-the-board will cause issues, so the best course forward is probably to add this as an option. It only _really_ matters if the date is being stored for utilitarian and not cosmetic purposes (e.g. schedule a cron task, as opposed to simply outputting a date string without timezone).

Anyway, I'd like an outside opinion on this. If we decide to move forward with this PR as-is, I'll add unit tests.