Support for async/await

[lmammino]

Coming out from a discussion in #49, it might be very useful to support async functions as middlewares.

The idea is that we might support middlewares with the following syntax:

export default function() {
  return {
    before: async (handler) => {
       // await async stuff
       return 'somevalue'  // equivalent to calling next(null, 'somevalue')
    }
  };
}

[lmammino]

Closed by #51