Promisify: Convert callback-based APIs to promises

Promises are a popular solution to some of the drawbacks of the
callback-style async APIs dominant in node.js libraries. But it's
awkward to write an node.js application using promises when all the
libraries you want to use are callback-based.

Hence Promisify. It converts callback-style APIs to use promises
instead. To use it, you provide a concise description of the
structure of the API to tell Promisify what to convert. For example,
here is a sample which converts a substantial portion of the MongoDB
driver API to Promises:

var promisify = require('promisify');

var promisify_collection = promisify.object({
insert: promisify.cb_func(),
remove: promisify.cb_func(),
findOne: promisify.cb_func(),
find: promisify.func(promisify.object({
stream: promisify.func(promisify.read_stream())
}))
});

var promisify_connection = promisify.object({
collection: promisify.cb_func(promisify_collection),
createCollection: promisify.cb_func(promisify_collection),
dropCollection: promisify.cb_func()
});

var promisify_mongodb = promisify.object({
connect: promisify.cb_func(promisify_connection)
});

var mongodb = promisify_mongodb(require('mongodb'));