Changes in Version 1.6

  o made nberShade() generic and added a method for ggplot objects

  o added fortify.tis(), a tis-specific method for the fortify() generic in ggplot2

Changes in Version 1.5

  o added options to how nberDates() and nber.xy() handle an ongoing recession

Changes in Version 1.4

  o fixed nberDates() to show latest recession

Changes in Version 1.3

  o cleaned up some documentation to incorrect package references and
    references to Fed-specific stuff.

  o added "tif" parameter to today() with default value "daily"

Changes in Version 1.2

  o better handling of ti objects with some NA values.  Thanks to Chuck
    Mikolajczak for pointing out the problem.

  o the observed attribute of a tis can now be one of "beginning", "ending",
    "summed", "averaged", "annualized", "high", or "low".  The last two are new,
    and denote series for which only the high or low value over a period is
    recorded. The convert() function can work with the new attributes.
