* chacha20poly1305 support

* add chacha20poly1305 backend and some fixes

* refactor

* forgot to remove this

* pep8

* review feedback and a lot of type/value checking

* review feedback

* raise unsupportedalgorithm when creating a ChaCha20Poly1305 object

if it's not supported.

* switch to ciphertext||tag

* typo

* remove a branch we don't need

* review feedback

* decrypts is *also* a word

* use reasons

* Don't raise an UnsupportedExtension for critical extensions.

Fixes #2903
Fixes #2901
Fixes #3325

* Don't link

* Revert "Don't link"

This reverts commit 4fe847f91d9dd45cdc28a4984c4e44aad62a5de6.

* fix

* Revert "Revert "Don't link""

This reverts commit 856031b5a1fbad04ac218fa94ebf37dcd402f3ed.

* fix

* Deprecate this

* Better changelog entry

* minor int_to_bytes performance increase

* why is Python 2.6 supported anyway...

* keep py2k version

* Remove API deprecated in 1.6, clean up the legacy deprecations

* flake8, unused import

* add support for update_into on CipherContext

This allows you to provide your own buffer (like recv_into) to improve
performance when repeatedly calling encrypt/decrypt on large payloads.

* another skip_if

* more skip_if complexity

* maybe do this right

* correct number of args

* coverage for the coverage gods

* add a cffi minimum test tox target and travis builder

This tests against macOS so we capture some commoncrypto branches

* extra arg

* need to actually install py35

* fix

* coverage for GCM decrypt in CC

* no longer relevant

* 1.8 now

* pep8

* dramatically simplify

* update docs

* remove unneeded test

* changelog entry

* test improvements

* coverage fix

* add some comments to example

* move the comments to their own line

* fix and move comment

* fix a regression in int_from_bytes

* add a new test file

* clean up int_from_bytes

7x speed-up and code is more readable

* remove unused import

* rely on py2 built-in codecs