Bump version string to 0.92

This lets Couchbase Lite tell that it supports gzipped request bodies.
Bug-fixes for compressed/encoded HTTP responses

For #238
Compress JSON doc bodies in multipart HTTP responses

But only if the request contains a nonstandard HTTP header:
X-Accept-Part-Encoding: gzip
because earlier versions of Couchbase Lite (as well as CouchDB) would
barf on this.
For #238
Defensive parsing of _revisions property in _bulk_docs requests

Fixes #241
Minor changes

* _changes now sets the correct application/json Content-Type
* Added a ?since= query param to the _dumpchannel request
Merge branch 'master' into faster_changes