sentry package

Included Raven, the Sentry client for php. Small readme contains more details, but you just have to
```php
Sentry::log('Your log message'); 
Sentry::exception($e);
```

You should include the package in you config file. In production mode when the package is loaded, it will override the default error handlers (the ones Fuel already overloads itself) and send errors to sentry. This can be disabled in the config file. The flag there doesn't affect on `DEVELOPMENT`, where error handlers are not touched.

Please review my code! I just included the Raven client (inside `vendor`), create a wrapper class with two static methods and that's it. Maybe we need more?

PS: the dns/user key for Sentry is my personal one for now, the free plan. We should change this.