Before & After commit hooks

Adds callbacks for creating/created/updating/updated models on a
controller.

Callbacks can be defined in controllers that inherit the
`EloquentController`.

``php
class UsersController extends EloquentController
{
    protected function creating(Model $model)
    {
        // Before the model is created.
    }

    protected function created(Model $model)
    {
        // After the model has been created.
    }

    protected function updating(Model $model)
    {
        // Before the model is updated.
    }

    protected function updated()
    {
        // After the model has been created.
    }
}
``

---

See #11 for a bit more discussion.`

src/Http/Controllers/EloquentController.php

@@ -244,7 +244,45 @@ protected function hydrate(ResourceInterface $resource, Model $model)
      */
     protected function commit(Model $model)
     {
-        return $model->save();
+        $isUpdating = $model->exists;
+
+        $this->beforeCommit($model, $isUpdating);
+
+        $result = $model->save();
+
+        $this->afterCommit($model, $isUpdating);
+
+        return $result;
+    }
+
+    /**
+     * Determines which callback to use before creating or updating a model.
+     *
+     * @param Model $model
+     * @param bool $isUpdating
+     */
+    protected function beforeCommit(Model $model, $isUpdating)
+    {
+        if ($isUpdating) {
+            return $this->updating($model);
+        }
+
+        return $this->creating($model);
+    }
+
+    /**
+     * Determines which callback to use after a model is updated or created.
+     *
+     * @param Model $model
+     * @param bool $isUpdating
+     */
+    protected function afterCommit(Model $model, $isUpdating)
+    {
+        if ($isUpdating) {
+            return $this->updated($model);
+        }
+
+        return $this->created($model);
     }
 
     /**
@@ -350,4 +388,48 @@ private function doDestroy(Model $model)
             return $this->destroy($model);
         });
     }
+
+    /**
+     * Called before the model is created.
+     *
+     * Child classes can overload this method if they need to do any logic pre-creation.
+     *
+     * @param Model $model
+     */
+    protected function creating(Model $model)
+    {
+    }
+
+    /**
+     * Called after the model has been created.
+     *
+     * Child classes can overload this method if they need to do any logic post-creation.
+     *
+     * @param Model $model
+     */
+    protected function created(Model $model)
+    {
+    }
+
+    /**
+     * Called before the model is updated.
+     *
+     * Child classes can overload this method if they need to do any logic pre-updating.
+     *
+     * @param Model $model
+     */
+    protected function updating(Model $model)
+    {
+    }
+
+    /**
+     * Called after the model has been updated.
+     *
+     * Child classes can overload this method if they need to do any logic post-updating.
+     *
+     * @param Model $model
+     */
+    protected function updated(Model $model)
+    {
+    }
 }