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
Fixed issue #8039: Problems with subquestion code '0'

DEV: Both described issues were separate incidences of
DEV: the same thing: php empty()/($var) treat the '0'
DEV: as not set or empty.
DEV: isset() on the other hand also returns "" as true.
DEV: I am afraid this problem may be present more often in
DEV: the code. Maybe implement a custom function
DEV: is_set() which also returns FALSE for "" but TRUE for '0'
New feature #8040: STATA export functionality

DEV: Implements XML-export for statistical software STATA
DEV: Thanks to mdekker for his support regarding the plugin
DEV: architecture.
Merge pull request #104 from mfaber/stata4

New feature #8040: STATA export functionality
dev: allow quick & dirty adding of plugins to the export dialog (to be improved)
Fixed STATA export: No output when debug!=0

DEV: Menno, both reported issues should be fixed with this commit.
DEV: Thanks for the close look!
Merge pull request #105 from mfaber/updatestataexport

DEV: Fixed STATA export: No output when debug!=0
dev: changed width of plugin grid and made layout more compact and bordered
dev: Moved export styles to plugin 
dev: used authdb since that is always enabled, we should move all core plugins that can not be deactivated here to prevent clutter in the plugin screen
dev: still todo: move r and spss here and update remote control
dev: moved plugins to demo so they don't show up and are only available for coders as inspiration since they serve no real purpose
dev: Added housekeeping to pluginmanager -> now missing plugins will be removed from database
Changed feature: Moved R-export to plugin and to normal export responses dialog
dev: a little more compact layout
dev: Fixed error where survey --> answers array not initialized
darkblue admin theme for color-impaired with screenshots
Fixed issue : Ranking question are not side by side in IE7
Dev: IE7 round up
Dev: I don't see this first time ?
Dev: trying overflow hack too : no way