added basic/digest support option

done for [AGIOS-255](https://issues.jboss.org/browse/AGIOS-255)

the functionality is exercised in the ```HttpAuthenticationTests``` test case class where the different http/digest options are executed plus a test case with  where a protection space is used and assigned to the NSURLSession.defaultConfiguration() (without the need to pass any credential object)

besides the tests, a demo app exerting basic/digest has also been created to demonstrate and can be found [here](https://github.com/aerogear/aerogear-ios-cookbook/pull/50)