Fix breaking changes to pycaffe

I broke pycaffe in #1014 (sorry!), because 88221c9a45120bd892180444c4d6ee9c41841079 removed `name` properties that were used internally in `pycaffe.py`. This PR returns everything to working order (checked against our examples) by exposing `Net::blob_names` and `Net::layer_names`, keeping the names internally accessible in Python from the same functions as in C++.