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
I've used ```$collection->getOnly```, there is some precedent for this in Google Guava: http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Iterables.html#getOnlyElement(java.lang.Iterable,%20T)