Include first driver: SHT31

Make I2C reads blocking w/ timeout
Add SHT31 driver, roughly :)
Fix mg_time() to return a double.

include/mgos_mock.h

@@ -30,6 +30,8 @@ int log_print_prefix(enum cs_log_level l, const char *func, const char *file);
   } while (0)
 
 
-float mg_time();
+double mg_time();
+
+void mgos_usleep(uint32_t usecs);
 
 #endif // __MGOS_MOCK_H