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')
You do this in two places. Could be replaced by a reusable function for clarity.

Also instead of concattenating strings with +, formatting is usually cleaner: `realdirpath = '%s://%s/' % (settings['proto'], urlparts.netloc)`.