Reset GRBL on open, to get version string

Universal G-Code Sender does not send any Reset command to GRBL, so GRBL does not answer with a version string, which prevents normal operations: sending g-code returns "GRBL has not finished booting", Soft Reset does nothing (since we don't know version). I've inserted code to send reset command on comm open, to get the version string as early as possible. This has the consequence that stopping and restarting Universal G-Code Sender would make GRBL reset, but otherwise it was not very functional...