reworked log plots. Default to symbol.

The default marker is now a symbol.

Factored out the code to deal with log plots and plot ranges.

Allow type= as a shorthand, with the marker type determined by the
value.  The other shorthands are dealt with when the maker is created,
e.g. Points or Curve

Allow scalar input

Documented the changes
document allowing scalars
rewrote plot to use ScatterPlot

Created a simple wrapper class ScatterPlot.  This is now used
by plot() to make scatter plots.

Also fixed bug where the default xrange and yrange were calculated but
not getting set for log plots.

exported ScatterPlot, but should debate if this is useful generally
do not expore ScatterPlot
fixed docs
Merge pull request #34 from esheldon/master

rework log plots, default to symbol, allow shorthand type=