Update scheduled tasks if definition changed

With this patch, when we update the command or the user, and if we set
"force" to true, then the scheduled task is updated.

BTW, I see in https://github.com/opscode-cookbooks/windows/blob/master/CONTRIBUTING that I should open a Jira task to contribute. However, according to http://www.getchef.com/blog/2014/06/16/chef-issue-tracking-and-contribution-workflow-updates/ I believe that this file is outdated. Please feel free to tell me If I'm wrong.
I think it'd be simpler to name the method ```task_needs_update?``` and then put it directly in the conditional, i.e. ```if task_needs_update?``` then you can drop a could above lines.