Additional fields in log msg via Java sys-prop

We have a need to populate additional fields in logstash messages with data from the environment, and names & values of these fields may change from environment to environment.

The most straightforward method we came up with is to add a system-property that the JSONEventLayoutV1 class inspects for these fields.

For example, one could pass -Dnet.logstash.log4j.JSONEventLayoutV1.UserFields=key1:val1,key2:val2 when calling java to start an application.  JSONEventLayoutV1 would then inject "key1":"val1", "key2":"val2" into every json log message.

This pull request only applies to JSONEventLayoutV1, as that is the layout we're using.  Also, I am unsure of the convention of naming system-properties, and assumed that it should be prefixed with the package name.