Extend support for `attachments` query parameter

Until now, the boolean query parameter `attachments` has only been
supported for the document API endpoint (`/{db}/{docid}`).

This extends support for queries to the changes (`/{db}/_changes`) and
view (`/{db}/_design/{ddoc}/_view/{view}`) API endpoints:  If both
`include_docs` and `attachments` equal `true`, the Base64-encoded
contents of attachments are included with the documents in changes or
view query results, respectively.

Closes COUCHDB-1923.