Analytics-python

- Fix the missing `upload_size` parameter

- Fix linting code and readme heling basic things.
- Add support for HTTP proxy
- Allows more settings to be configured from singleton

- Add `sync_mode` option ([147](https://github.com/segmentio/analytics-python/pull/147))

- Add User-Agent header to messages
- Don't retry sending on client errors except 429
- Allow user-defined upload interval
- Add `shutdown` function
- Add gzip support
- Add exponential backoff with jitter when retrying
- Add a paramater in Client to configure max retries
- Limit batch upload size to 500KB
- Drop messages greater than 32kb
- Allow user-defined upload size
- Support custom messageId

- [Fix](https://github.com/segmentio/analytics-python/pull/102): Stringify non-string userIds and anonymousIds.

- [Fix](https://github.com/segmentio/analytics-python/issues/94): Date objects are removed from event properties.
- [Fix](https://github.com/segmentio/analytics-python/pull/98): Fix for regression introduced in version 1.2.4.