Upgrades (#14)

* enable tracing

* more config adjustments

* upgrading images

* upgrading input

* upgrading led

* upgraded pins api

* upgrade runtime

* upgrading pxt.h

* upgrading basic.cpp

* upgraded control

* upgraded control.ts

* upgrading core.cpp

* updating shims

* fixing merge issues

* upgrading BLE config

libs/core/pxt.h

@@ -160,6 +160,7 @@ namespace pxt {
     inline void unref()
     {
       //printf("DECR "); this->print();
+      check(refcnt > 0, ERR_REF_DELETED);
       refcnt -= 2;
       if (refcnt == 0) {
         destroy();