MemoizeObject implementation

So here's my initial check in of the ```@MemoizeObject``` annotation. I believe all we need to do is add in that gson plugin as far as the dependency stuff goes. I haven't had a lot of experience actually packaging up the plugin so any guidance in that regard will be welcomed. I added that dependsOn to the grails plugin file to try to enforce it.

I added in an integration test as well to prove out that it's working. I updated the documentation as well. It's important to note that the Object class needs to be able to be able to be converted to JSON and back. The gson plugin was giving me some problems with a really messy class that had generic Enums fields and other craziness going on. For straight forward simple classes it should work fine though. I tried to point that out in the documentation but I'm up for adding more if needed.

Let me know what you think of it so far. Thanks!