Enable dot syntax to get field from belongsTo model

[mrk-j]

Possible fix for #46, maybe @jspekken can check this out?

It works like this:

Model

<?php
class Model
{
    public function reference()
    {
        return $this->belongsTo('ReferencedModel');
    }
}

Admin

<?php
class ModelAdmin extends Admin
{
    public function configureList($mapper)
    {
        $mapper->identifier('reference.field');
    }
}