Support for multiple files

(Awesome project! I was partway through writing my own code when I thought to Google and see if anyone else had already! This is my first ever pull, so sorry if I'm not doing it right.)
Added support for multiple files. Moved main logic inside the argc loop--not the nicest, but works as long as files are last argument. Also added filename to output line. 
Sample:

```
./find-face --cascade=haarcascade_frontalface_default.xml *.jpg
face_2012-01-10T23-00-00Z-0800.jpg 0.425 0.352083 0.304688 0.40625
face_2012-01-11T07-30-25Z-0800.jpg 0.390625 0.4625 0.282813 0.377083
face_2012-01-11T10-00-00Z-0800.jpg 0.503125 0.0166667 0.301562 0.402083
face_2012-01-11T20-00-00Z-0800.jpg 0.898438 0.36875 0.071875 0.0958333
face_2012-01-11T20-30-00Z-0800.jpg 0.534375 0.433333 0.128125 0.170833
face_2012-01-11T21-00-00Z-0800.jpg 0.364062 0.285417 0.395312 0.527083
```