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.

{
    QSettings settings;
    
    BackupSettings(GetDataDir(true) / "guisettings.ini.bak", settings);
    
    QString dataDir = Intro::getDefaultDataDirectory();
    dataDir = settings.value("strDataDir", dataDir).toString();
    
    settings.clear();
    
    settings.setValue("strDataDir", dataDir);
    
    settings.setValue("fReset", true);
    
    if (GUIUtil::GetStartOnSystemStartup())
        GUIUtil::SetStartOnSystemStartup(false);
}
int OptionsModel::rowCount(const QModelIndex & parent) const
{
    return OptionIDRowCount;
}
struct ProxySetting {
    bool is_set;
    QString ip;
    QString port;
};
static ProxySetting GetProxySetting(QSettings &settings, const QString &name)