Python wrapper fixes and re-arrangement

N.B. this PR is against `master` to address users' current problems, and reduce the number of issues raised that are better solved by examples and documentation.

This is a placeholder to let everyone know I'm working on (list in progress):

* formatting helpers to convert images to Caffe format for deployment, or convert Caffe images back to normal images for viewing
* batching helper for feeding inputs to the net with padding as needed
* python exceptions instead of library crashes 
* a more complete wrapper example that shows how image formatting, computing the forward pass,
and reading off predictions works
* properly separate responsibilities between the pycaffe wrapper, the wrapper example (wrapper.py) and detection example (detector.py)