Extracts Simperium::Auth out into its own file.

* Adds the WebMock gem for testing so we do not have to make external 
  API calls.
* Adds tests for Simperium::Auth using Minitest. Minitest will run by 
  default in 1.9+. To run the tests in 1.8 you will need to install the 
  minitest gem.
* Small refactorings to Simperium::Auth

TODO: Still requires testing for Error Handling.