Anti-spam

Hi Matt,

I use your static comments plugin in [my Jekyll blog](http://digitalwords.net/) (thanks!). I moved to Jekyll yesterday, and haven't got any real comments yet, but I did get A LOT of spam.

For solving this, I modified plugin a little: when writing a comment, one has to type a short string which will be checked against a constant in the PHP file (normally just copying some words, but if automatic spam bots pass this ‘Turing test’, change the instruction into something trickier…).

I hope you will incorporate this change in the main branch or as a secondary branch ☺

Have an interesting day,
Júda