Merging eslint/eslint#master to andreypopp/eslint#jsx

Not being able to install your port using npm 2.x (because of an old version of optionator depending an a module only in pre-release), I realised you were a few months behind eslint's master.

I merged it and send it back to you, as-is. There was only one significant conflict, and I was obviously unable to fix it :( so I'm not sure if "no-unused-vars" rule works with jsx. It may be easy for you to fix it again, and have an up-to-date fork :)