Makes entry points return some information

The setup function defined by the entry_point mechanism now returns a
list of dicts, which each contain information about a specific unpacker.
The "official" entry_point have been updated to return that information,
which currently includes:
  - 'name', the readable name of the unpacker (that is the argparse
    command group)
  - 'test_compatibility', which indicates if the unpacker will work on
    the current machine with the given pack file

This will be useful to list and test the available unpackers.
Merge branch 'unpacker-info' into 'info-command'

Conflicts:
	reprounzip/reprounzip/reprounzip.py
Makes 'reprounzip info' print compatible unpackers
Simplifies test_compatibility
Simplify test_compatibility protocol
Merge branch 'unpacker-info' into 'info-command'