unban command, ban duration, default value for ban duration in config

[0x17de]

Unban users based on their IP. No server restart required anymore.
Default ban duration in configuration file is 1 hour and can be set to "permanent" to go with the old style ban system.

Now users can be banned for a specific duration via:
send({cmd:'ban', nick:'test'});
send({cmd:'ban', duration:'1s', nick:'test'});
send({cmd:'ban', duration:'1m', nick:'test'});
send({cmd:'ban', duration:'1h', nick:'test'});
send({cmd:'ban', duration:'permanent', nick:'test'});

And those can be unbanned via:
send({cmd:'unban',ip:'127.0.0.1'});

Those changes were tested on a private server.