Re-index child documents using _parent field

The parent field mapping is defined on a child mapping, and points to
the parent type this child relates to.
In case on child type re-indexation, we should explicitly request for it
within fields parameters, then provide
the value for indexing.
Re-index child documents : bug fix and tests

. MySearchResponseJson.java : fixed bug (I forgot to specify _parent
field within scan search)
. ReIndexActionTester.java :  added reindexChilds function
Merge pull request #19 from djanssen/master

Re-index child documents using _parent field