Response Validation & Processing

- [x] Add Response Precondition checking
- [x] Add Response signature verification
- [x] Add Response Validation
- [X] Add Response Assertion Decryption
- [x] Add Response Assertion Validation
- [x] Add Response Assertion Transformers
- [ ] Test all the things
Well, SAML2 is full of collections that 99% of the time contain no more than 1 element (like Assertion, multiple assertions is interop suicide) so a shorthand that not only gives the first Assertion but also verifies that it is the only one would be very helpful.