[pycaffe] expose Net::CopyTrainedLayersFrom as Net.copy_from

This allows finetuning from Python, for example.
[pycaffe] expose Net::SharedTrainedLayersWith as Net.share_with
Merge pull request #1195 from longjon/python-copy-from

Expose Net.copy_from and Net.share_with in pycaffe
fix instantiation