Feature/Sort fields and parent mapping

[vrcmarcos]

Sort: https://www.elastic.co/guide/en/elasticsearch/guide/1.x/multi-fields.html
Parent mapping: https://www.elastic.co/guide/en/elasticsearch/guide/current/parent-child.html

[lauxley]

hi, thx for the pr !
I just did a quick pass on the code and it looks cool but i have a few remarks:

• would it be possible to make a PR for each feature ? (sort fields and parent mapping)
• I'm wondering if parents mapping isn't out of scope for this package, what problem does it solve ?
• no test for sorting fields