Add support for built-in couchdb reduce functions in map reduce specs

couchdb provides a few simple reduce functions for common numeric operations, which are written in Erlang and run inside couchdb for performance (see https://wiki.apache.org/couchdb/Built-In_Reduce_Functions). Specifying these like `view ..., :reduce => "_sum"` already worked fine in couch_potato, but this PR adds support for them in map reduce specs by making equivalent Javascript versions available.