Initial support for RAW requests

I've made an attempt for supporting RAW requests, how does this look like? I'm sending the pull request so that we can comment on it, the code is not finished yet, nor have I tested to see if it breaks anything (yet :smile: ).

In order to be able to expect a structured reply (XML, JSON) as a result of a RAW request, I've added a new step (?), `Given reply format "{format}"` which lets the user specify the expected format. If this is not specified, the same format of the request is assumed, e.g. request JSON, expect JSON reply.