Refactor TestContext to a standalone class.

Modify AsyncTestCase.createRestAdapter() to use a TestContext
instance instead of the null result of getActivity();
Preserve REST accessToken in SharedPreferences
Add getApplicationContext() and getRestRepository()
Implement UserRepository.currentUserId

The id is updated by login/logout methods and preserved in
SharedPreferences across app restarts.
Add findCurrentUser, getCachedCurrentUser
Merge pull request #32 from strongloop/feature/preserve-current-user

Store current user id & access token, add API for accessing the current user