Filters throgh relations

Adding support for filtering data through relationships. Good examples of this can be found in the file `test/associations/multiple-level-filters.test.js`

Here is the added behavior: 

```js
Task.findAll({
  where: {
    'project.user.username': 'leia'
  }
}).success(function(tasks){
  //...
});
```

By running this code, it sequelize will interpret the where clauses adding all the necessary joins to able to run the filter.

Hope you guys like it and consider the merging