IP validation bug

[SecretUsername]

An export with the following IP setup will never be possible atm:

IP: 10.0.0.1
Net: 10.0.0.0/16
Gateway: 10.0.255.254

Same for e.g. IP 10.0.1.255.

I would recommend that the validation in pluginlib.py should calculate the broadcast address instead of assuming 255 will be the right choice in either case.