change axis hash with object
move method : from Chart#convert_axis_args to Axis#merge_with_hash
Axis : remove prefix "_" from all instance variable.
axis.rb : remove unused method.
Axis#merge_with_hash : @args changed to args because it is used only in this method.
remove Chart#write_val_axis_common. It isn't used.
add Axis#write_number_format which used from Chart#write_number_format.
Axis#write_number_format : changed to receive @writer parameter.
add Axis#write_cat_number_format which used from Chart#write_cat_number_format
chart.rb : remove temporary variable 'attributes'
rename private method : Chart#get_area_properties -> Chart#area_properties
chart.rb : rename get_* method. remove get_