add output feedback mode support + test vectors (aes)
change OFB iv to nonce to reflect dstufft nomenclature pitch

* Namely, we should try to call things IV if reuse leaks a small amount
  of data and nonce if reuse can result in a complete break. This can
  be somewhat ambiguous, but we'll track in #58
Merge pull request #57 from reaperhulk/ofb-support

Output feedback mode support + test vectors (aes)