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.

{
    if(fState)
    {
        ui->minimizeToTray->setChecked(false);
        ui->minimizeToTray->setEnabled(false);
    }
    else
    {
        ui->minimizeToTray->setEnabled(true);
    }
}
void OptionsDialog::showRestartWarning(bool fPersistent)
{
    ui->statusLabel->setStyleSheet("QLabel { color: red; }");
    if(fPersistent)
    {
        ui->statusLabel->setText(tr("Client restart required to activate changes."));
    }
    else
    {
        ui->statusLabel->setText(tr("This change would require a client restart."));
        
        
        QTimer::singleShot(10000, this, SLOT(clearStatusLabel()));
    }
}
void OptionsDialog::clearStatusLabel()