support extra (required) S3 pre-signed headers

This adds initial support for extra signed headers that can be used in a pre-signed `PUT` URL. The extra headers supplied by the client at upload time.
Actually the  extra `'x-amz-acl': 'public-read'` field I took from the test just previous to the one I added. It does not have anything to do with testing this `extraHeader` and in fact I have no idea what it would do given that the function being called (`client.signedUrl`) for the parameter called `options` is never used in the function (see [here](https://github.com/LearnBoost/knox/blob/master/lib/client.js#L858) ).