Add secrets.example.php, make dnsbls.php optional

This is an alternate solution to pull #134.

A file, secrets.example.php, is added to serve as an example for the secrets.php file, which is now required.
The DNSBL file is made optional.

The database configuration, the tripcode salt, and the cookie salt are in the example secrets.php file.

Advice to generate the salts from the kernel's random number generator is included, as that will give ~120 bits of entropy.