add date boundaries to the salary object

[hancush]

historically, bga has gathered prospective pay for a standard period (the calendar year). someday, they may gather actual pay. on that occasion, because employees start or leave jobs at all times of year – not just jan 1 or dec 31 – it would be nice to keep track of bounding dates for that pay. (see #56 / #57.)

[hancush]

the future is now. bga is gathering actual pay for 2017, this year (2018). the responding agencies are not providing start and end dates explicitly. however, if a start date is provided (historically, about half of payroll records have them), and it is the same as reporting year, we'll have the start date for the salary. agencies are not providing an end date, but some agencies are indicating where pay is only for part of the year, which is better than nothing.