Added a provider for Phalcon

Added a provider for Phalcon. The provider uses the default Phalcon DI unless you pass a DI into the constructor.

```php
new Whoops\Provider\Phalcon\WhoopsServiceProvider;

// --- or ---

$di = Phalcon\DI\FactoryDefault;
new Whoops\Provider\Phalcon\WhoopsServiceProvider($di);
```