Add support for Bulk get of documents

[Andrew-Lees11]

CouchDB has an API where you can request multiple documents are once:
http://docs.couchdb.org/en/2.2.0/api/database/bulk-api.html#db-bulk-get

We currently have support to Bulk insert or update documents but not to bulk retrieve them.

This API would be useful when you need to request lots of documents with known ID but don't want to make lots of API requests.

[Andrew-Lees11]

As a note, Bulk deleting documents should also be supported.