Add the ability to reload() a model, updating data for all references to that model

Previously, if you wanted to get updated data from the DB you would do like this:

````javascript
User.find(1).done(function (err, user1) {
  ...
  // someone else updated user, so it's now out of sync
  User.find(user1.id).done(function (err, user2) {
    // user2 is now up to date, but user1 is still out of sync
  })
})
````

This PR adds the ability to reload a model:
````javascript
User.find(1).done(function (err, user1) {
  ...
  // someone else updated user, so it's now out of sync
  user1.reload().done(function (err, user2) {
    // user2 is a reference to user1, so both are now up to date
  })
})
````