Use an user-friendly time data converter.
Cleanup imports.
Don't validate/convert time for whole_day.

Start/end-time for whole_day allocations are generated automatically and the start/end time fields
of an allocation are hidden in the user interface. Thus we should not validate user input in that case.
Merge pull request #3 from 4teamwork/friendly_time_data_converter

Improve Allocation/Reservation User interaction.