examples: rename MQTT-SN example for clarity

examples/emcute/Makefile → examples/emcute_mqttsn/Makefile

 # name of your application
-APPLICATION = emcute_example
+APPLICATION = emcute_mqttsn
 
 # If no BOARD is found in the environment, use this default:
 BOARD ?= native

examples/emcute/main.c → examples/emcute_mqttsn/main.c

@@ -11,7 +11,8 @@
  * @{
  *
  * @file
- * @brief       Example application for demonstrating the RIOT network stack
+ * @brief       Example application for demonstrating RIOT's MQTT-SN library
+ *              emCute
  *
  * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
  *