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.

{
    
    
    proxyType curProxy;
    if (m_node.getProxy(NET_IPV4, curProxy)) {
        proxy.setType(QNetworkProxy::Socks5Proxy);
        proxy.setHostName(QString::fromStdString(curProxy.proxy.ToStringIP()));
        proxy.setPort(curProxy.proxy.GetPort());
        return true;
    }
    else
        proxy.setType(QNetworkProxy::NoProxy);
    return false;
}
void OptionsModel::setRestartRequired(bool fRequired)
{
    QSettings settings;
    return settings.setValue("fRestartRequired", fRequired);
}
bool OptionsModel::isRestartRequired() const
{
    QSettings settings;
    return settings.value("fRestartRequired", false).toBool();
}
void OptionsModel::checkAndMigrate()