ER: Add crypto API for ChaCha20-Poly1305

[aburks]

Add an API for the ChaCha20-Poly1305 AEAD as defined by https://tools.ietf.org/html/draft-irtf-cfrg-chacha20-poly1305.

[toddouska]

Thanks for the request. We'll work on it this week.

[aburks]

I'll actually have a pull request for you tomorrow if you want to wait and have a look at that.

[aburks]

Added pull request #10 to fix this issue.

[toddouska]

Thanks for the pull request, merged.