Cleanup the javadocs on the FileBasedPkAuthenticator

Add more bits to the readme
Rename FileBasedPKAuthenticator to HomeDirectoryScanningPKAuthenticator which describes what the class actually does.
It scans /home looking for authorized_keys files for users
And from a security perspective, leaves the proxy pretty wide open
Probably should rename it, LargishSecurityHole.  ;)
Continue cleaning up docs and comments
Fix com.yahoo.sshd.authentication.file.MultiUserAuthorizedKeysMap.updateUser(String, InputStream) to close the inputstream
Merge pull request #19 from areese/cleanup_user_key_loading

Cleanup user key loading