Update SSHServer validate_public_key documentation

This commit updates the documentation for SSHServer's
validate_public_key to make it clear that the application only needs to
report that the public key provided is a valid client key for the
specified user. If it is, AsyncSSH will verify the private key
automatically.
Add links to the project's Github pages to the sidebar
Check in an initial set of unit tests

This commit updates setup.py to support running unit tests and adds an
initial set of unit tests focused on reading and writing SSH private and
public keys.