Add a .merge function

[nozzlegear]

Updating a document right now requires pulling the current document from the database, modifying it, and then sending the entire thing back through the PUT request with the revision. CouchDB supports an update function that we can use to merge partial documents into the original document and then return the new doc. It would need to be set up in a unique view, so Davenport should check that the view with update function has been set up before running the merge function.

https://wiki.apache.org/couchdb/Partial_Updates