Various minor fixes/improvements (#239)

Includes:
- Making "rpm_data_free" a public method and using it where appropriate
  - Includes fixing memory leak in epics demo application
    (note Valgrind indicates that there are more...)
- Fix typo in cov.c
- Allow NULL max_apdu parameter to address_get_by_device
- Add missing include to s_cov.h

apps/epics/main.c

@@ -299,7 +299,8 @@ static void MyReadPropertyMultipleAckHandler(uint8_t *service_request,
         } else {
             if (len < 0) { /* Eg, failed due to no segmentation */
                 Error_Detected = true;
-}
+            }
+            rpm_data_free(rpm_data);
             free(rpm_data);
         }
     }