Added helper to create linestyles

This commit adds a class Style to add linestyles to the plot. Usage

    require "gnuplot"

    Gnuplot.open do |gp|
      Gnuplot::Plot.new( gp ) do |plot|
        plot.xrange "[-10:10]"
        plot.title  "Sin Wave Example"
        plot.ylabel "x"
        plot.xlabel "sin(x)"
        
        # Create the style
        s = plot.style do |style|
            style.lt = 1
            style.lc = 3
        end

        plot.data << Gnuplot::DataSet.new( "sin(x)" ) do |ds|
          ds.with = "lines"
          # Use the style in a plot
          ds.ls = s
        end
      end
    end