Allow unsetting a gnuplot variable

This change adds a method Gnuplot::Plot.unset(var) to unset the Gnuplot variable var. As
the order of sets and unsets are important for Gnuplot, a new array "settings" was
introduced in which the set and unset tasks are accumulated.
Readme steps bug fix.
Bulleted lists fixes, wrapped all the inline code properly.
Oops, textile lists don't behave the expected way. Fixed.
Apparently inline code is harder than it looks.
Added linecolor property.
Merge pull request #13 from evnu/master

Allow unsetting Gnuplot variables
Merge pull request #15 from kot-behemoth/master

Readme fixes and linecolor property
v bump 2.6.0 thanks guys
Fix bug in SPlot.to_gplot implementation.
Make SPlot.to_gplot use parent's implementation
Merge pull request #16 from TheKnight/master

Bugfix for SPlot
v bump 0.6.1
Merge branch 'master' of https://github.com/rdp/ruby_gnuplot

Conflicts:
	lib/gnuplot.rb

Merge with branch master from rdp/ruby_gnuplot
Merge pull request #18 from llrt/master

Make SPlot.to_gplot use parent's implementation
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
Make 3d_surface_plot behave like the other examples

It was missing lib directory in the load path
Merge pull request #20 from rafaelrosafu/master

Minor fix on 3d_surface_plot example
Merge pull request #19 from zedalaye/patch-1

Exposed DataSet#index property
Add helper to create linestyles