Added a way to delete nested fields.
Empty out "more" field, but completely delete the "bytes" field.
Added a non-Ruby filter to delete nested fields.

Referenced by Jordan Sissel in:

    https://groups.google.com/d/msg/logstash-users/QWbd6loM9DY/McnqWcchO1gJ
Updated Ruby filter readme. There is a way to delete a field inside
Logstash.
Merge pull request #12 from shurane/delete_nested_field

Examples for deleting nested fields.