generalized -evalDetection

This pull request adds support to -evalDetection for galleries where the detection is stored in 4 separate metadata keys: `<key>_X` `<key>_Y` `<key>_Width` `<key>_Height`. This simplifies evaluating detectors like `Cascade(FrontalFace)` and addresses @bhklein's use case.

@bklare Can you review? Comments inline.