Change encrypt/decrypt doc functions name and signature.
Add MAC authentication to encrypted docs.

* Fix review comments:
    * Use of literal string instead of self.STORAGE_SECRETS_KEY
    * Add mac_method param to mac_doc()
    * Verify mac_method in mac_doc() and raise in there if unknown method
* Use different parts of storage_secret for generating doc passphrase and mac key.
* Add changes file.
Remove dependency on oauth
Fix pep8 style.
Merge remote-tracking branch 'drebs/feature/2349-add-mac-auth-3' into develop