Added new Xapiand client for Python, based on Elasticsearch client and improves aggregations.

Added

• Added new Xapiand client (based on Elasticsearch client)
• Added sorting for Aggregations
• Added _keyed to aggregations optional. Keyed are off by default now
• Added _shift to aggregations

Deprecated

• Schema deprecated string type, it now only has text and keyword (string will be removed at some point)