Move slack, hipchat and jabber integrations to plugins

doc/plugin-notifications.markdown

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+Add Notification Types with Plugins
+===================================
+
+You can send notifications to almost any system by adding a new type.
+There are two kinds of notifications: project and user.
+
+- Project: Notifications configured at the project level
+- User: Notifications sent individually and configured at the user profile
+
+Register a new notification type
+--------------------------------
+
+In your plugin registration file call the method `setType()`:
+
+```php
+$this->userNotificationType->setType('irc', t('IRC'), '\Kanboard\Plugin\IRC\Notification\IrcHandler');
+$this->projectNotificationType->setType('irc', t('IRC'), '\Kanboard\Plugin\IRC\Notification\IrcHandler');
+```
+
+Your handler can be registered for user or project notification. You don't necessary need to support both.
+
+When your handler is registered, the end-user can choose to receive the new notification type or not.
+
+Notification Handler
+--------------------
+
+Your notification handler must implements the interface `Kanboard\Notification\NotificationInterface`:
+
+```php
+interface NotificationInterface
+{
+    /**
+     * Send notification to a user
+     *
+     * @access public
+     * @param  array     $user
+     * @param  string    $event_name
+     * @param  array     $event_data
+     */
+    public function notifyUser(array $user, $event_name, array $event_data);
+
+    /**
+     * Send notification to a project
+     *
+     * @access public
+     * @param  array     $project
+     * @param  string    $event_name
+     * @param  array     $event_data
+     */
+    public function notifyProject(array $project, $event_name, array $event_data);
+}
+```
+
+Example of notification plugins
+-------------------------------
+
+- [Slack](https://github.com/kanboard/plugin-slack)
+- [Hipchat](https://github.com/kanboard/plugin-hipchat)
+- [Jabber](https://github.com/kanboard/plugin-jabber)
+