Feature: Enable PrettyPageHandler setup a specific layout

I need to have two different error messages:

1) For a development environment, rich of details, like PrettyPageTemplate is. 
2) Another for a production environment, containing just the error and some information for track back;

I plan call PrettyPageHandler like this:

```php
$handler = new PrettyPageHandler();
if (!Context::getInstance()->getDevelopmentStatus())
{
	$handler->setTemplateFile('layout_minimal.html'); 
}
```

To do this I have to change 2 files:

* PrettyPageHandler.php: adding the property $templateFile (getTemplateFile() and setTemplateFile())
* whoops.base.css: add 2 css styles.

And I added one file that is my suggestion for the "production" error layout.