Client and Client.MakeRequest porcelian

This commit brings in concept of a Cient struct that holds the auth
config and a http.client for issuing requests, and attaching porcelian
methods too.

* NewClient() method that accepts a name and key as strings for easy
  usage
* client.MakeRequest() method that is used for making arbitrary requests
  using the Client credentials. This method builds the http.Request and
  signs it under the hood.
Merge pull request #14 from go-chef/MakeRequest

Client and Client.MakeRequest porcelian