With a pruned wallet possible in master I believe we should expose the option in the GUI.

The Main tab in preferences will offer a chance to toggle the node type and size.
<img width="612" alt="pruning_gui_off" src="https://cloud.githubusercontent.com/assets/216917/8766147/834fb842-2e2c-11e5-94b0-c342132243bf.png">
<img width="612" alt="pruning_gui_on" src="https://cloud.githubusercontent.com/assets/216917/8766148/8351a0c6-2e2c-11e5-841b-af02fd77ff44.png">

The prune size UI should not permit sizes less than 550.

{
    static const ProxySetting default_val = {false, DEFAULT_GUI_PROXY_HOST, QString("%1").arg(DEFAULT_GUI_PROXY_PORT)};
    
    if (!settings.contains(name)) {
        return default_val;
    }
    
    QStringList ip_port = settings.value(name).toString().split(":", QString::SkipEmptyParts);
    if (ip_port.size() == 2) {
        return {true, ip_port.at(0), ip_port.at(1)};
    } else { 
        return default_val;
    }
}
static void SetProxySetting(QSettings &settings, const QString &name, const ProxySetting &ip_port)
{
    settings.setValue(name, ip_port.ip + ":" + ip_port.port);
}
static const QString GetDefaultProxyAddress()
{
    return QString("%1:%2").arg(DEFAULT_GUI_PROXY_HOST).arg(DEFAULT_GUI_PROXY_PORT);
}
QVariant OptionsModel::data(const QModelIndex & index, int role) const