Version 0.7
    o bug fix saving declared data frames as .csv files
    o as_declared() now retains the "format.spss" attribute, if existing

Version 0.6
    o improved treatment of arithmetic operations, attributes are no longer
      propagated (thanks to Randall Pruim for the suggestion)
    o modified as_haven() for objects of class 'declared', to avoid a bug in
      package ReadStat that prevents exporting missing values to SPSS for
      integer variables

Version 0.5
    o initial version
