Date_picker: Generate HTML form inputs to pick dates

This class can generate HTML code for form inputs to let the user pick a date.

It can:

a) automatically selects a the correct pulldown values from a MySQL record field in the "date" format of "0000-00-00".

b) Re-selects the previously or newly chosen values if the form is re-sent (as during data validation errors)

c) Lets you choose the initial date, or today's date as the default

d) Lets you choose the minimum and maximum year values or lets you choose a difference value for the minumum and for the maximum as well when populating the year pulldown