Auth changes

For your information: the changes I made to enable (basic and) ldap authentication.

Note: to use this, you need:
     sudo apt-get install python-paste
     sudo apt-get install python-ldap

Note that I run this via stunnel, set-up via the ~/screenly/misc/enable_ssl.sh script provided with screenly.
Note that when you enable stunnel using that script, you have to change the firewall rules to open port 443, by issuing: sudo ufw allow 443/tcp
Probably this should be fixed in the ~/screenly/misc/enable_ssl.sh script.

(Edited to include 'sudo apt-get install python-ldap')
Better to use a proper logging channel here. E.g. with Python [logging](https://docs.python.org/2/library/logging.html#logging.Logger.error) you might write, `logger.error("Cannot use %s authentication in web interface, because the username and/or password is not set in the Screenly configuration file.", settings['auth'])`.

print() is best reserved for quick debug output IMHO.