Added InResponseTo and ACS handling to the StubIdp

Added the possibility to unbind the Saml2RedirectBinding
Added missing Saml2RedirectBinding_Bind test
Add more tests

Fix 68
Change acs URL in the Stub IDP to sp.example.com as in other examples in the code

Also added display of InResponseTo (if present) in the Stub IDP
Refactor: Convert AssertionModel.Default property to AssertionModel.CreateFromConfiguration() method as per discussion in https://github.com/KentorIT/authservices/pull/69/files#r13321348
Convert the InResponseTo label to a manipulatable field that is available all the time as per discussion in https://github.com/KentorIT/authservices/pull/69/files#r13321374
Reformat code
Improve informative text now when not all requests are unsolicited
Add test for generation of the InReponseTo attribute in the Saml2Response xml
Refactor: move reading of properties belonging to the base class from Saml2AuthenticationRequest to Saml2RequestBase
Merge pull request #69 from albinsunnanbo/68SetInresponseToInStubIdp

Add InResponseTo and ACS handling to the StubIdp.
- Merging this even though code coverage drops below 100%. There is finished code that will fixed it and will be submitted as a PR as soon as this one is merged.