drivers/nrf24l01+ : added netdev implementation for nrf24l01+ transceiver

This PR is based on #1704. It contains a connection between the lowlevel part for the nrf24l01+ transceiver-driver and the netdev interface. I assume there will be several deficits, especially because this device fulfills no standard. So let's discuss this!
Hey!! :(
```diff
diff --git a/drivers/include/netdev/default.h b/drivers/include/netdev/default.h
index 50d4983..d044a93 100644
--- a/drivers/include/netdev/default.h
+++ b/drivers/include/netdev/default.h
@@ -36,4 +36,12 @@
 #endif /* NETDEV_DEFAULT */
 #endif /* MODULE_NATIVENET */
 
+#ifdef MODULE_NRF24L01P
+#include "nrf24l01p.h"
+
+#ifndef NETDEV_DEFAULT
+#define NETDEV_DEFAULT   ((netdev_t *)(&nrf24l01p_netdev))
+#endif /* NETDEV_DEFAULT */
+#endif /* MODULE_NRF24L01P */
+
 #endif /* __NETDEV_DEFAULT_H_ */
```