Refactor OAuth config (again)

Previously we were only saving some of the information we needed.  This makes a few changes:

* Renames rsa_key to consumer_secret
* Saves access_secret in addition to access_token
* Adds a JiraCards::OAuth::access_token which creates an
  OAuth::AcessToken object using data from the config file
* Refactor load/save methods so they can be tested
* Update CLI to match new OAuth/Config changes