Xbee 802.15.4 modem driver for Arduino

This radio driver allows to use an XBee 802.15.4 modem with an Arduino board, using the official Arduino Wireless/SD shield (see http://arduino.cc/en/Main/ArduinoWirelessShield) for connection.

With regards to #1492 this would be a struct extending `net_dev_t`

```C
typedef struct {
    net_dev_type_t type;
    net_dev_802154_driver_t *driver;   // net_dev_802154_driver_t is a net_dev compatible rewrite of your ieee802154_driver_t
    uart_t uart_link;
    gpio_t sleep_rq_gpio;
    gpio_t on_status_gpio;
} xbee_net_dev_t
```
and this way be configured by the user at runtime.