Support building with prebuild CouchDB

This branch is the result of several hours getting this plugin installed into a prebuilt CouchDB.

It includes a Makefile, and one change to include a dependency via rebar.

I tested with OSX 10.9 and `brew install couchdb` but it should work with other packaged builds (ubuntu, official).

Telling people to use `build-couchdb` is not very friendly when it rebuilds Erlang on every run.