Fixed smile detection example

The example for smile detection has a few errors.
1) When creating the detector, you need to pass in CIDectorTypeFace not CIDetectorSmile.
2) You need to pass in CIDetectorSmile as an option to `featuresInImage:options:` to actually turn on smile detection (it's performed as an additional step when finding faces).
3) Once you have a CIFaceFeature, you need to check the hasSmile property to see if a smile has been detected. In the sample, I'm just checking the first face.