Add cli cron system

This adds a cli cron system.
Each cli_*.php script can now have an optional getCronInfo() method, which returns an array which describes how frequently(in seconds) to run this cli with which arguments. The arguments can either be a string or an array. If it's a string, the string is exploded on " " to get the indivudial arguments.

Additionaly, it parses $base/cron.overrides (which is a json formated file), and overrides all commands that are in there.
A possible cron.overrides file could be:
```json
{
    "apiFetch": {
        "300":""
    },
    "parseKills": {
        "300":""
    },
    "stompReceive": {
    }
}
```
This would disable stompReceive entirely, and run apiFetch and parseKills every 5 minutes instead of every minute.

As the result, the only crontab that needs to be added now is running cron.php every minute.

The script needs the pcntl php extension, to fork of childs which continue to run in the background.