Fix an error when setting the legend position to 'none'

When setting the legend position to 'none' the gem crashes because it tries to access the "legend" var instead of the existing "@legend" initialized by the "set_default_properties" method.

Note that setting the legend position to 'none' still has no effect on the graph (the legend is still visible at the bottom).
Hide the legend if the @legend[:visible] params is set to 0

Instead of check for a boolean expression, check for the numerical 0
add test for issue25  -  bug in Chart#set_legend
Merge branch 'patch-1' of https://github.com/almathie/writeexcel into fix_issue25

Chart#set_legend(:position => 'none') make chart legend invisible.
add examples/chart_legend.rb
README.rdoc : add v1.0.1 comment, and delete old changelog.
set version to v1.0.1.