Adds 'reprounzip info' command
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'
Merge branch 'master' into 'info-command'
Makes tests/run.py run 'reprounzip info'
Checks for Linux before comparing distrib/arch
Adds a readable "not POSIX" error message

Previously, unpacking with 'directory' or 'chroot' would fail when
constructing a Path from a PosixPath was wrong (like on Windows).
Changes plugin entry_point protocol

This changes the API with which plugins register with reprounzip.

The name of the entry_point is now used as the name of the reprounzip
subcommand (e.g. 'reprounzip graph') and the declared entry_point
function takes the subparser directly as parameter. The function's
docstring is used as help message.

These setup functions might return either None or a dictonary containing
information on the command (this will be useful later, e.g. to display
which plugins are compatible).
Merge branch 'change-entrypoint-protocol' into 'info-command'

Conflicts:
	reprounzip-vagrant/reprounzip/unpackers/vagrant.py
	reprounzip/reprounzip/main.py
	reprounzip/reprounzip/unpackers/default.py
	reprounzip/reprounzip/unpackers/graph.py
Merge branch 'info-command' into 'master'

Conflicts:
	reprounzip-vagrant/reprounzip/unpackers/vagrant.py
	reprounzip/reprounzip/unpackers/common.py
	reprounzip/reprounzip/unpackers/default.py
	tests/run.py