Don't hash the input hostname when checking a host key if the input hostname is already hashed

This happens in the loop checking if a hostkey is already loaded when
loading host key files. This reduces time to load host keys file with
about two seconds for me
Changelog re #272
Set Transport.active to False early in Transport.__init__.
Changelog re #312, closes #312
Fix some minor typos.
Changelog re #324, closes #324
Updating doco for sftp_client.put()

Adding a note to indicate that the remotepath should include a filename.