New feature #7810 Implement max/min date for date question

DEV: implemented min/max dates for datepicker in Date question
DEV: supported formats/dates:
DEV: - fixed dates in format YYYY-MM-DD (e.g. 2013-08-24)
DEV: - {qcode}: reference to another datefield
DEV: -      supported on same (dynamic, JS) and on previous pages (PHP)
DEV: -      honors survey date format and date formats in
DEV: -      advanced attributes of a date question (mixing date
DEV: -      formats in one survey should be no problem)
DEV: - {expressions}: only on following page
DEV: -      Things like date from previous answer+3 days as min
DEV: -      for another question is possible using eg.
DEV: -      {date("Y-m-d",strtotime(departure)+3*60*60*24)}
DEV: -      on a following page
DEV:
DEV: I recycled the dropdown_dates_year_min/max fields
DEV: to avoid cluttering up the user interface. The fields still
DEV: take fixed years in format YYYY for backward compatibility,
DEV: so old survey will work as they used to.
DEV:
DEV: This also fixes #7981 (bug in jquery UI) because we do not use
DEV: the yearrange function of the datepicker anymore,
DEV: which is affected by this bug.
DEV:
DEV: Tested in FF and Chrome
renamed dropdown_dates_year_min/max

need to make DB-conversion and add to import_helper
Update DB for name change of attrib fields
Make sure min/max year from old lss-files are imported
fix for year=0 in dropdown-boxes

When dropdown boxes are used and a min/max value refers to an empty
variable
or zero value, default range is now used
Merge pull request #96 from mfaber/datepicker5

New feature #7810: Implement max/min date for date question