Added option for excluding fields through options on the server side.

Helpful if you like to exclude sensitive fields from your public API

Example usage would be:

````js
var User = require("../../models/user");

var express = require("express")
var app = module.exports = express()

var restify = require("express-restify-mongoose");

restify.serve(app, User, {
  prefix: "/api",
  exclude: {password: 0, _id: 0, email: 0, ip: 0}
});
````

Cheers!