Remove Couchbase.save({'_id':'key', 'json':'doc'}) in favor of .set() doing key name detection

It seems .save() was added to do some special handling (_design/doc PUTing). It leans on doc._id which is going away in 2.0, so it "smells" if nothing else.

Additionally, leaning on .set() and cb['bucket']['key'] = 'value' for the API will give us great consistency and less confusion.

.save() should be deprecated
.set() should use JSON by default
.set() should handle keys named '_design/' via the HTTP API