ADDED:
+ stubbed out the io stuff for the timesheet files
+ stubbed out the model for timesheets -- there'll just be one table in
	the .timesheet files that just holds a bunch of stamps:
		id, start_time, stop_time
	I expect the model to be pretty dirt simple.

REFACTOR:
+ the MainWindow now takes a handle to a TimesheetIO object that it uses
	to do create, save, close, etc.

FIXED:
+ updated the README