Improved Model base class

Examples:

```php
// Attempts to create the Model_Post object
$model = Model::factory('post');

// Attempts to create the Model_User object
$model = Model::factory('User');

// Attempts to create the Model_Collection_Document object
$model = Model::factory('collection_document');

// Attempts to create the Document object
$model = Model::factory('\Document');
```