Changes plugin entry_point protocol

I don't like the way plugin setup functions have to return a list of dicts with a 'name' key in them, on branch info-command.

From [.../unpackers/default.py@784259d9](https://github.com/ViDA-NYU/reprozip/commit/784259d99ec6a411511859d009635a22c6fcff92#diff-5f383d98cb6278864e6c48037d17a3f5R272):
```python
    return [{'name': 'installpkgs', 'test_compatibility': test_same_pkgmngr},
            {'name': 'directory', 'test_compatibility': test_linux_same_arch},
            {'name': 'chroot', 'test_compatibility': test_linux_same_arch}]
```

This branches makes non-backwards-compatible changes to the plugin "API". There is now one setup function per command, and its docstring and the entry_point name are used by reprounzip to create the subparser beforehand. Hopefully this will make info-command cleaner.