Allow marshaling and unmarshaling with io.Writer and io.Reader

[mfcochauxlaberge]

Currently, one needs the whole slice of bytes before unmarshaling and then marshaling returns a complete slice of bytes.

For performance reasons and to deal with a http.Request.Body in a way that makes more sense, there should be a way to unmarshal from an io.Reader. For similar reasons, there should be the option to marshal to an io.Writer.