fix old detect.py default
pycaffe Detector crops with surrounding context

- caffe.Detector learned how to crop windows with context in the R-CNN
  style s.t. the bordero of the network input is a given amount of
  context.
- add --context_pad arg to detect.py for amount of context. Default is
  16, as in R-CNN.
make R-CNN the Caffe detection example