Update demo.py

why import threading?
Store hostname hashes in memory rathen than the non-hashed host entries. Also assures that the host entries in known_hosts get saved in hashed format as it is currently standard in OpenSSH.
Assure that host entries in known_hosts files do not duplicate endlessly if keys from known_hosts are loaded via HostKeys.load() more than once (e.g. for refreshing the list of known hosts during runtime).
Load host entries from the known_hosts file(s) before writing the file from RAM to disk. Avoids loss of host entries in case other SSH clients have written to the known_hosts file(s) meanwhile.
do not write ,,garbage'' to known_hosts file(s)
Changelog re #87
Back out broken but non-required hostname hash change
Fix indentation at few places.
Changelog re #146 (also start 10.10.2 section)
Merge branch '1.10'

Conflicts:
	NEWS
Warn on parse failure when reading known_hosts
Merge pull request #153 from glasserc/log_bad_hostkeys

Warn on parse failure when reading known_hosts
Warn on parse failure when reading known_hosts
Merge branch '1.10'
Merge pull request #155 from Baconator507/patch-1

Update demo.py
Add explicit link to updated API docs. Fixes #160
Merge branch '1.10'
Merge branch 'master' of github.com:paramiko/paramiko
Bump dev version
Flip bad known_hosts line to INFO from WARN re #153