Add cognitoAuthenticated getter to determine whether session is active

[joefresco]

Currently, it is impossible to determine if a session has timed out. This adds a getter that validates the session expiration against the current time. amazon-cognito-identity-js uses the cognitoSession class to do this, which would be ideal, but I think this approach works.

[ffxsam]

@kesor Just curious, is this repo still maintained? I'd like to contribute, but only if PRs will actually get reviewed and merged.

[kesor]

@ffxsam yes, but at the moment I don't have enough time to add unit tests to PRs. and I don't want to merge commits without unit tests.