GZip-compress HTTP request bodies, when server supports it

Compressing requests is somewhat nonstandard. There’s no built-in way to
tell if the server supports it (unlike compressing responses, where the
server can check the request’s Accept-Encoding header.) So yet again we
check for a specific min version of Sync Gateway to decide whether to
enable it.
For #240
Compress outgoing doc JSON bodies in multipart format

For #240
Decode compressed JSON MIME bodies in multipart server responses

Support Gzip Content-Encoding of a document body encoded in a MIME
part (as when downloading a doc with attachments.)
For couchbase/sync_gateway#238