compound datetime, wrapper that allow editing datetime using two fields.

One for date, and one for time.
has_compound_datetime to ease using CompoundDatetime
Test posting event start_time using compound_start_time_attributes
Use CompoundDatetime to edit event start and end time
Use js datepicker to input date
validates that end_time >= start_time
Adjust compound datetime input style
simple_form zh-CN translation should use key zh-CN