Improve documentation

[tpetrone]

It would be nice to explicitly inform that when in test environment is mandatory to use the following options:

    ENV['ember-simple-auth-token'] = {
      ...
      refreshAccessTokens: false,
      tokenExpirationInvalidateSession: false
    };

Otherwise later() (Ember's run loop function) will cause acceptance tests to halt and eventually time out depending on token's lifetime (server side) and test's timeout configurations.

[fenichelar]

@tpetrone Good point! I will add this :)

[fenichelar]

Closed by #253.