Changes in Version 1.9

  o convert() uses tapply() which calls as.factor() on a ti object. For some
    reason, this results in NA's in R 2.10.0.  Unclassing the ti before using
    it as a parameter to tapply fixed it.  Why factor() gives NA's for ti
    objects is a subject for further investigation.

Changes in Version 1.8

  o Allow fortify.tis to handle proper naming when a univariate array is 
    converted into a tis object.  Contributed by Trevor Davis.

Changes in Version 1.7

  o Tweaked nberShade.ggplot to work with a greater range of ggplot objects,
    and improved examples for nberShade and fortify.tis documentation.
    Contributed by Trevor Davis.

Changes in Version 1.6 (contributed by Trevor Davis)

  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.