Fix two issues with Python 3:
* Use class annotations to denote an implementation of a
  Zope interface;
* A classical byte/str issue.

The previous name incenp.certbot-ssh was needlessy using both a
dot and a dash. Since the name must be used on the command line
when calling Certbot, it's probably best to keep it simple.

Rename the project to incenp.certbot-ssh (place it under "my"
incenp namespace); update the setup.py file and add encoding
stanza to the ssh.py module.

Add NOTICE file and make sure to how use original license file
as freshly downloaded from apache.org; also rename the license
file to LICENSE, as recommended by the ASF.

Move the helper script inside the incenp.certbot package. This
ensures it will be picked up by setuptools using the
'include_package_data' option.

Add support for pkgutil-style namespace packages (for 'incenp').

Add the location of the project's homepage and repository.

The .gitignore still refered to the old letsencrypt-ssh name.

PEP8 recommends all lowercase for package names.

We do not require the ssh-server option to be set at the parsing
stage, as the parser would then raise an error even when certbot
is not called to perform any validation (e.g., when the `plugins'
command is used to list plugins).

Instead, we use the `prepare' step to check if the option has
been set, and raise an error if appropriate.

Make sure the README is up-to-date after the Letsencrypt-to-Certbot
change.

Letsencrypt was renamed to Certbot by the EFF. This plugin could
still function without any changes (Certbot developers seemed to
have taken great care to avoid incompatibility issues caused by
the name change), but let's avoid relying on that and make the
necessary update.

When validating a domain, append a line to the system log
giving the validated domain and the corresponding challenge.

Provide a sample script to run on the remote server. It assumes
each domain has a webroot directory under /var/www/domain.

This is letsencrypt-ssh, a plugin for the Let’s Encrypt client
to perform HTTP01 challenge validation on a remote computer
through a SSH connection.