Upgrade plugin and sample app to grails 2.3.6

I was having trouble getting the example app working on grails 2.2.4 (kept complaining about a mismatch with the slf4j jar files), so I upgraded the plugin and the example app to the latest version of grails, 2.3.6.

I updated the tests so they are all passing.

I also slightly tweaked the `PegController` in the example app so that the actions that make sense to be parameterized (histogram and timed) allow the user to pass a value in to generate a different value.

I have a few other ideas for enhancements to make to the plugin (mostly around letting the user automatically set up timing for controller and service methods), but if I get to them, I'll submit them in a separate pull request.