[aerogear-ios-http] Hook for authentication challenge

As per the discussion started in:
https://github.com/aerogear/aerogear-ios-http/pull/73

We should provide a way to hook into the delegate method (closure or sth equivalent) to manage different cases in authentication challenge while doing basic/digest auth.
Read more: http://pjeremymalouf.com/managing-an-nsurlsession-authentication-challenge-in-swift/