import_spss() and meta data on time, date and datetime variables

[beckerbenj]

Currently, haven does not support any meta data on time, date and datetime variables (missing tags and value labels are silently dropped in read_sav()). import_spss() translates these variable types to character and uses an internal workaround to apply all missing tags (via times2character()).

However ideally, missing tags and value labels should be preserved and eatGADS should generally support datetime formats (via POSIXlt).