CLS shimmer for Mongoose

Adds CLS wrapper for Mongoose, making mongoose safe to use with continuation-local-storage.

Usage:

``js
var cls = require('continuation-local-storage');
var clsMongoose = require('cls-mongoose');

// or cls.getNamespace if exists
var clsns = cls.createNamespace('app');

var mongoose = require('mongoose');
clsMongoose(clsns);
`

After mongoose` is patched, use it as usual.