More convenient methods for token expiration

Instead of just determining if a token as already expired, you can now
also ask if a token is about to expire within a specified number of
minutes.
Ability to setup the current user from existing

You can now setup the CurrentUser by providing an already existing
Auth0User object. This is useful for when you store the user details on
the device in a secure way so that you can restore it later on.
Built against specific versions of dependencies