luatz: Move `asctime` from timetable.lua to strftime.lua. It doesn't deserve to be a timetable method
luatz/timetable: Move require call to top of file
luatz/init: Add gettime and tzcache to exported table
luatz/init: Add C-like functions for some common tasks
spec/strftime_spec: Use the same timestamp every test run.
spec/strftime: Generate format strings before tests, and use in test names
luatz/tzcache: Allow absolute paths