[TASK] Allow directory as argument for db:dump

Added an is_dir check to support the following syntax:

```
rick@rick-desktop:~/public_html/magento/web$ ./n98-magerun.phar db:dump ../

                       
  Dump MySQL Database  
                       

Filename for SQL dump: [../2013-12-14_210711_magento.sql]
Start dumping database magento to file ../2013-12-14_210711_magento.sql
Finished
```