Version 0.5

-- PlotItem has been renamed to DataSet and all of the subclasses have been
renamed as well.

-- There's a fairly fundamental change in the way that DataSets are created
from an object. In the past, there was a giant switch statement that was used
to map a class to its associated data type. Now, using Ruby's mechanism to add
methods to existing classes, you need to define a gpds (GnuPlot DataSet)
method to return a DataSet instance of the appropriate type.

In order to facilitate this, when the Gnuplot file is loaded, Array, Matrix,
String, and NArray will have the gpds method added.  This method returns a new
instance of the Gnuplot::DataSet object that is initialized with the receiver.


-- PlotBase has been removed and Plot is now the base class.  Splot now
derives from Plot.

-- Beginnings of some unit tests (bad coder for not doing this earlier).

-- Demos have been broken out into their own files.


