Exposed DataSet#index property

Use of index property is required for me to be able to draw a curve based on a simple dataset of values (x, y) :

dataset = Gnuplot::DataSet.new( [x, y] ) do |ds|
  ds.with = "linespoints"
  ds.notitle
  ds.using = "1:2"
  ds.index = 0
end
Merge pull request #19 from zedalaye/patch-1

Exposed DataSet#index property