Response Validation & Processing

- [x] Add Response Precondition checking
- [x] Add Response signature verification
- [ ] Add Response Validation
- [ ] Add Response Assertion Validation
- [ ] Add Response Assertion Transformers
- [ ] Test all the things
Hehe, good point. Remnant of trying to figure out what was going on [here][1]...

[1]: https://github.com/simplesamlphp/simplesamlphp/blob/master/modules/saml/lib/Error.php#L42