Object/Class method support for plugins

This pull request adds support for objects and class methods within plugin hooks.

Example:

```php
class MyPlugin
{
	public function postbitHook(&$post)
	{
		$post['someNewContent'] = 'Hello World!';
	}
}

$myPlugin = new MyPlugin;

$plugins->add_hook('psotbit', array($myPlugin, 'postbitHook'));
```

This allows for a more object orientated approach to plugin development while maintaining backwards compatibility with plugins using traditional functions.