Extract Simperium::Api to its own file.

* Create a spec_helper.rb file to bundle up required spec files.
* Small refactorings to Simperium::Api.
* Specs for Simperium::Api.
Remove Simperium::Admin since it doesn't seem to do anything.
Extract SPUser to its own file.

* Moves SPUser to its own file.
* Adds methods to fetch the userid and username.