bring back padding layer with deprecation notice

- bring back padding layer
- split into cpp, cu
- add deprecation notice
bring back padding test and lint it
style: line continuation spacing
Merge pull request #227 from shelhamer/padding-deprecation

Bring back padding layer to ease release upgrade
Add comment explaining placement of system headers in C++ Python module
Raise Python exceptions if CaffeNet input files don't exist

This is for convenience only; if the input files are moved or deleted at
the wrong time, pycaffe could still crash (as it did before). However,
in most cases this will make interactive use easier.