The SubscriptionThrottle plugin limits how fast users can subscribe to groups
and other users.

Installation
============
add "addPlugin('SubscriptionThrottle');"
to the bottom of your config.php

Settings
========
subLimits: Array of time spans in seconds to limit subscriptions to users.
groupLimits: Array of time spans in seconds to limit subscriptions to groups.

Example
=======
addPlugin('SubscriptionThrottle', array(
    'subLimits' => array(
        86400 => 100,   // 100 subs per day
        3600 => 50;     // 50 subs per hour
    ),
    'groupLimits' => array(
        86400 => 50,    // 50 subs per day
        3600 => 25;     // 25 subs per hour
    )
));