Just Pushed: Improve source based fields loading when searching

Here is the issue:
https://github.com/elasticsearch/elasticsearch/issues/1347. It explains the
current problem when fetching fields from source, and it fixes all of them.
Specifically, _source. prefix appending to the result field name,
automatically extracting objects as well as actual fields (without the need
to prefix with _source, and actually, it is recommended not to do it
anymore), and better inner arrays handling.