XTS support
add the actual xts tests
refactor XTS to require split keys

* First half to AES object, second half to XTS
* Add split_key clsasmethod on XTS
* Revert AES object to proper key_sizes
* Add initial docs (improvements welcome)
additional key data should be required, checked for size
expand explanation for split key in docs
remove xts large key test (unnecessary with split key approach)