Allow to setup a parent document on the ES ouputs

*** This is an updated version of the previous #505 Pull Request ***

Add the parent document setting on the 'elasticsearch_http.rb' and
'elasticsearch_river.rb' outputs. I have also unified the way to create the ES
message in these two outputs.

'elasticsearch.rb' has not been changed as 'jruby-elasticsearch' (the ES
library used in that output) doesn't seem to have the concept of
'parent' or at least I haven't found it.