Hook method to add custom data to JSON event

There are certain situations where you want to include custom data in the produced JSON event. This may include e.g. the argument array, unformatted message, etc.

Rather than including explicit support for all these situations, this pull request adds a hook method to JsonLayout, enabling you to simply subclass and override the new method and add the fields you need to include in the produced JSON.

An example where this would be useful is described here:
http://stackoverflow.com/questions/22615311/is-there-a-logback-layout-that-creates-json-objects-with-message-parameters-as-a