add Shape class
Extract Sparkline class done.
Extract method : Sparkline#initialize was extracted from Worksheet#add_sparkline. and remove '_' from head of instance variable of Sparkline.
move method : from Worksheet#attributes_from_sparkline to Sparkline#group_attributes
Sparkline#initialize : use each to set_spark_color
Sparkline : remove unused attr_ menbers.
Sparkline : require utility.rb
add Sparkline#count
Sparkline : some modify
Merge branch 'add_sparkline_class'
sparkline.rb : modify comment.