Replace bin/* with an entry_point dict in setup.py
 - All main functions can be called with no args or a single arg,
   which should be a list of arguments
 - Simplified some tests
 - All entrypoints should support -h, --help, some support 'help'
Fixed failing 'dt last week'
 - Implemented tests for 'last week', 'next week' in test_dt
 - Fixed dt.py to satisfy the new tests