Start refactoring on client/context

* Add resource Context used to store relations, an optional response and
  used to resolve relative URIs. Similar to a web page context.
  Will be used in future to store e.g. Accept-Headers etc.
* Move HTTP logic to Restify::HTTP in order to extract into a distinct
  obligation-powered HTTP client gem later.
* Drop Resource#status and Resource#code to not override common used
  attributes with response values.
* Resource#follow also follows Content-Location now.
* Relation always uses full URI for template but can be compared (#==)
  with actual value in Link or attribute (e.g. relative path)