Cache the complete body

  - Implement the raw_get method to get the full response body, as the
    timestamp encoded are quite useful in conjunction with cache.
  - Caching is done on the above method.
  - The original get method just calls raw_get to extract the result.
Make available the timestamp in some way.

  - Set the API object to contain the timestamp of the last called
    retrieval.  Cached responses should return the same timestamps
    associated with it.