Add support for sending commands to device

First implemented command is clearNotifications to remotely clear
already read notifications. Like the notifications the command is
also sent encrypted with user password.

For the script this adds a new option called
`irssinotifier_clear_notifications_when_viewed` to clear notifications
on the device when the hilights have been seen in irssi.

The clear notifications command is sent after user changes away
from the last window that had hilights.