AES Counter support

* vectors from RFC 3686
* Documentation for the mode
update docs to roughly describe many time pad attack & link cbc
remove confusing explanation about reusing nonce in counter mode docs
ctr doc language take 3
additional doc work on ctr
Merge pull request #104 from reaperhulk/ctr-support

AES Counter support
Merge branch 'master' into refactor-cipher-names
Include CTR mode