Added check before executing a class method other than build.

Line 638: method_exists($this,$this->params['controller_method'])) is necessary in order to avoid loading inexistant methods from the current class instance, and moreover, any class instances invoked when loading them as a module (using $this->addModule()), defaulting to build.

Eg: Using the new router, we want to run update method.
```php
class HomeIndexController extends \Sifo\Controller
{
   function update($id)
   {
     //...
     $this->addModule('flash_message','HomeFlashMessage');
     //...
   }
}
```

```php
class HomeFlashMessage extends \Sifo\Controller
{
   function build()
   {
     //...
   }
}
```
$this->addModule will invoke HomeFlashMessage. If class is not modified, the current code will try to run the "update" method on HomeFlashMessage, resulting in an error. This patch defaults to build() as update() doesnt' exist.
Merge pull request #21 from nilopc/patch-3

Added check before executing a class method other than build.