WIP: log query in auto-explain

I put the attached together a while ago and neglected to send it. 
Basically it includes the text of the query being explained in the 
explain output.

I think it's more important to include the query text when using 
auto_explain than in other explain uses, which is why I did it this way. 
But I could be persuaded that we should always include it.

Thoughts? Comments?

cheers

andrew