add EspClass::getFreeHeap see #81

Author: Links2004

cores/esp8266/Esp.cpp

@@ -70,3 +70,10 @@ uint16_t EspClass::getVCC(void)
 {
     return system_get_vdd33();
 }
+
+uint32_t EspClass::getFreeHeap(void)
+{
+    return system_get_free_heap_size();
+}
+
+

cores/esp8266/Esp.h

@@ -44,6 +44,7 @@ class EspClass {
         void reset(void);
         void restart(void);
         uint16_t getVCC(void);
+        uint32_t getFreeHeap(void);
 };
 
 extern EspClass ESP;