TO DO

CHANGES IN VERSION 0.2.4

* plotting functions make full use of the dots argument (...); formal arguments
  ylim, xlab, ylab, etc, removed.

* densityPlot and overlapPlot gain an 'extend' argument: if not NULL, the
  plot is extended by 3 hrs each side of the focal 24-hr period and 'extend'
  specifies the background colour to use; if NULL, the plot is not extended.

* overlapPlot gains a rug argument (suggestion of Anand Roopsind)

CHANGES IN VERSION 0.2.3

* overlapPlot and densityPlot gain argument xcenter, which controls whether the
  the plot is centered at noon (default) or midnight (wish of Bronwyn Fancourt).

CHANGES IN VERSION 0.2.2

* overlapPlot gains argument ylim (wish of Jesse Lewis)

* densityPlot gains arguments ylim, xlab, ylab (wish of Wenhong Xiao)

CHANGES IN VERSION 0.2.0

* Fixed bug in overlapPlot (reported by Jesse Lewis) and added overlapPlot to
  examples on 'kerinchi' help page.
  
* overlapPlot gains arguments linewidth, xlab and ylab.

