Make CouchBase.request() more generic

[Jason Gerard DeRose]

At this point Microfiber is an extremely nice HTTP client, and we might as well use it for things besides just talking to CouchDB.

First on my list is to use it for Dmedia file download and upload. I don't care about Microfiber *directly* supporting this in a nice way. I just want to be able to create a CouchBase subclass and add some custom methods, but still use the same authentication and connection reuse and all that.

The issue is CouchBase._request() current reads the response data and returns a (response, data) tuple. For file download, Dmedia needs to call response.read() itself as it write leaves into the file.