Define application name through the configuration.

Defines the application name through the configuration array (just in __construct). For example:

```php
$app = new Slim(array('name' => 'Foo'));
$app->getName(); // outputs "Foo"
```

This would avoid a setName call after instantiation. Maybe a __construct parameter would suit better, I'm not sure.

PS: As requested by Josh, now I'm doing a pull request to the "develop" branch.

Thank you.