Anti-spam measures

In response to spammers using the board, I have put some changes in place:

1) I have disallowed guest postings. I did so reluctantly because I'd prefer it if anyone could comment freely without needing to register but this facility was being abused.

2) Registration now requires entry of a 'captcha' code. This involves reading some text from an image and entering it into a box, a task which is easy for humans (as long as they have reasonable eyesight) but hard for the programs used by spammers. Anyone who does not have sufficiently good eyesight will need to contact me in order to get registered.

3) The membership list has been removed. This is a step recommended by anti-spam experts.