Flanker

Changed
- Replace the leading '.' in an quoted-printable encoded mime part to avoid
obscure SMTP bug

Changed
- Support for Python 3 was added with preserving the Python 2 behavior in mind.
As a result Python 3 specific logic may be not that efficient due to extra
conversions between text and bytes, but that is left for future improvements;
- CRLF is now consistently used when a parsed mime is serialized into a string.
- Dependency on cchardet was made optional. Ported from [PR84](https://github.com/mailgun/flanker/pull/84)
- [PR94](https://github.com/mailgun/flanker/pull/94) Local Redis cache was made
configurable via environment variables REDIS_HOST, REDIS_PORT, and REDIS_DB
with the defaults matching the original behavior.

Changed
- MAX_HEADER_LENGTH (8000) and MAX_LINE_LENGTH (10000) parser limits were
removed.
- A bunch of implementation details were "hidden" in underscore prefixed
files/folders. This change is technically breaking, but those files were
never supposed to be used directly.

The change log has not been kept up to release v0.8.5. The only reliable
source of information is the commit log on [GitHub](https://github.com/mailgun/flanker/commits/master).