Allow format_date smarty function date parameter to be a well formatted datetime string

This PR allows the smarty function format_date to get a string as date parameter.
String formats can be found [here](http://php.net/manual/en/datetime.formats.compound.php)
example:
```smarty
{format_date date="2014-08-25 08:12:34"}
```

	modifié:         core/lib/Thelia/Core/Template/Smarty/Plugins/Format.php