Removed node-sass-middleware as a dependency

We've already pulled all the functionality into https://github.com/sass/node-sass-middleware, but having it as a dependency in package.json makes for some weird circular dependencies.

Seeing as we're going 1.0.0, I don't have a problem breaking backwards compatibility a little.

I've left an error message to guide people in the right direction, we can drop that in a future version.