Add support for sql comments

It would be great to be able to annotate sql with comments to tell where a query came from/or user/account runs it -> visible in slow query log and mysql status

-> SELECT * FROM users WHERE xxx /* user.rb:11 */
User.where(xxx).comment("doing slow stuff in the console: do not kill")

does this sound good -> would a patch be accepted / did anyone already build something similar ?

I would accept support in ARel, but I'm not sure about adding the
`comment` method to AR::Relation. We'll have to discuss it more, but
the first step is to put it in ARel.