add simple User model testing

tests/models/User_spec.js

@@ -0,0 +1,41 @@
+process.env.NODE_ENV = 'test';
+
+var User = require('../../models/User');
+
+var mocha = require('mocha')
+  , chai = require('chai')
+  , expect = chai.expect
+  , mongoose = require('mongoose');
+
+var user;
+
+describe('User attributes', function() {
+  before(function(done){
+    user = createUser();
+    done();
+  });
+  
+  it('_id is a mongoDB ObjectId', function() {
+    expect( user._id ).to.be.an.instanceOf(mongoose.Types.ObjectId);
+  });
+  
+  it('email should be a string', function() {
+    expect( user.email ).to.be.a( 'string' );
+  });
+})
+
+function createUser(){
+  var user = new User({
+    email: getEmail(),
+    password: '__password__',
+  });
+  user.save();
+  return user;
+}
+
+// create unique email address
+// 
+// TODO - use an implementation that guarantees string uniqueness
+function getEmail(){
+  return new Date().getTime() + '@example.com';
+}