Add development webserver with proxy to CouchDB

This add serve command which is a simple HTTP static file server of attachments dir with integrated proxy to CouchDB based on the rewrites specified in app.js .

IMHO this makes development even simpler that sync, and also avoids lots of file uploads to the DB, especially if you code messy like me with a lot of small changes and experimenting.

Basic usage is :

    couchapp serve app.js http://localhost:5984/example_db

Now you can access your couchapp at http://localhost:3000/. When you think it is good enough for production :

    couchapp push app.js http://localhost:5984/example_db