Release: NavBot rev. B

Firstly, thanks to everyone who's posted or written to me to tell me you like NavBot! It makes me happy :-)

Ahem. Secondly: Sorry to everyone who's had problems with revision A. Just for you, I've put together a quick revision B with the same features but extra error reporting. I've opened up the public comments on this page, so please post your error logs here and I'll try to work out what's going on!