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.

#ifndef BITCOIN_QT_OPTIONSMODEL_H
#define BITCOIN_QT_OPTIONSMODEL_H
#include <amount.h>
#include <QAbstractListModel>
namespace interfaces {
class Node;
}
QT_BEGIN_NAMESPACE
class QNetworkProxy;
QT_END_NAMESPACE
extern const char *DEFAULT_GUI_PROXY_HOST;
static constexpr unsigned short DEFAULT_GUI_PROXY_PORT = 9050;
/** Interface from Qt to configuration data structure for Bitcoin client.
   To Qt, the options are presented as a list with the different options
   laid out vertically.
   This can be changed to a tree once the settings become sufficiently
   complex.
 */
class OptionsModel : public QAbstractListModel
{
    Q_OBJECT
public:
    explicit OptionsModel(interfaces::Node& node, QObject *parent = 0, bool resetSettings = false);
    enum OptionID {