#410 Artifact cleanup extension

@sriki77's idea for initialization was to use the ```@Load``` and ```@UnLoad``` annotations. They are implemented already. I was thinking about those. In that case, the GoPlugin (API endpoint) will be instantiated and any ```@Load``` methods on it will be called. The code is [here](https://github.com/gocd/gocd/blob/master/plugin-infra/go-plugin-activator/src/com/thoughtworks/go/plugin/activation/DefaultGoPluginActivator.java#L154). I was wondering whether it would make sense to use it.

Whichever plugin does not need access back to the Go Server does not need to implement the method at all. If it needs it, then, it can implement ```@Load``` and get the PluginContext object, which can have the ```GoApplicationAccessor```.

I'm thinking about it since it's a more common initialization mechanism across plugin types.