Fix minute formatting

Use a two digit format.
Make _get_unique_items() more idiomatic

Use Python's standard library instead if the obvious hack.
Less code repetition in get_report()
Implement archive() command
Housekeeping

Cosmetic changes, docstrings.
Catch empty log file
Install as 'tk' to PREFIX/bin

Also make tk script executable.
Add .gitignore
Fix booking time spent at given dates
README cosmetics