cascade classifier(CvHaar..) and objdetect module

Added `CascadeClassifier` (`CvHaarClassifierCascade`) and the `objdetect` module (just the `cvHaarDetectObjects` method). I'm pretty certain the `cvHaarDetectObjects` method is from the older opencv version and is deprecated but I couldn't find any newer `C` methods for this..

Wasn't sure what to name the _safer_ version of `CvSeq` (or should I have just implemented an `Iterator` for `CvSeq`?) so I just named it `Seq` (`src/core::Seq`) but feel free to merge and change it to whatever fits.
Add `use ffi::core::*;` and remove `::ffi::core::`.