Migrate core middleware to goa-middleware

[derekperkins]

I think that all the middleware should exist in one place, either here in the main goa repo or over in github.com/raphael/goa-middleware. As is, there are a few "blessed" middleware that are a part of the main package.

The most compelling reason I can think of to include middleware in goa is if there is any need to share types through an internal package.

[raphael]

Done, all middlewares are now at https://github.com/raphael/goa-middleware.