Hash hostname patch revisited

Hi Jeff,

this is a follow-up of #88. Unfortunately, the patch in #88 had an error that caused the observed trouble.

This pull request contains the patch with the earlier error fixed. Below you find some help text for the changelog file...

"""
Add method kwarg ,,hash_hostname'' to hostkeys.HostKeyEntry.add() method.
Enable host key hashing by default as is standard with serveral current (Open)SSH implementations.
"""

Greets,
Mike