Added a tox/travis just for running flake8 under py3k
Merge pull request #206 from alex/py3pep8

Added a tox/travis just for running flake8 under py3k
Merge pull request #205 from reaperhulk/simplify-tests

Simplify some test class structure
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)
reorganize xts tests & add skip support