[models,configs/models] Cleanup model implementation to re-use all backbones from torchvision; Simplify checkpointer; Remove own implementation of model_zoo; Implement normalization for torchvision-based backbones; Enables pytorch installation on osx

[configs] Documented all configuration files; Added script to list/describe/copy configuration files; Re-structured user guide

[conda] Remove "license_file" keyword from recipe (addresses bob/bob.devtools#44, see bob/bob.devtools!127)