PostgreSQL hashed password authentication

Not sure if there is any interest in this, but I added an auth method based on hashed passwords stored in a PostgreSQL database. It requires the pgcrypto module (part of postgresql-contrib in Debian, and requires running `CREATE EXTENSION pgcrypto;` on your database server. To set up a user you can do something like the following:

```
CREATE TABLE users(handle VARCHAR(255), domain VARCHAR(255), pw VARCHAR(255));
INSERT INTO users (handle, domain, pw) VALUES ('username', 'example.com', crypt('password123', gen_salt('bf')));
```

If this is considered for merging with the main Radicale branch please let me know and I can write more thorough instructions and update the default config too.