Make detection module for python wrapper

This promotes `power_wrapper` to its own `detection` module, generalizes it as needed for use with other networks and data sets, and renames it `detector`. The further aim of this refactoring is to pave the way for other caffe-derived detectors.

- Generalize the hardcoded and ImageNet-specific aspects of `power_wrapper` to ease the use of different architectures and data sets (such as Pascal VOC)
- Default settings to the imagenet configuration for the sake of the demo
- Promote `power_wrapper` to its own submodule `caffe.detection` as `detector`