Stravalib

- Fixed update_activity to include description (hozn, 91)

- More Python3 bugfixes

- Python3 bugfixes (Tafkas, martinogden)
- Added delete_activity
- added context_entries parameter to get_segment_leaderboard method (jedman)

- Use (require) more modern pip/setuptools.
- Full Python 3 support (using Six). (hozn, 69)
- Webhooks support (thanks to loisaidasam) (hozn, 77)
- explore_segments bugfix (hozn, 71)
- General updates to model/attribs (hozn, 64, 73, etc.)

- Renamed `Activity.photos` property to `full_photos` due to new conflict with Strava API (hozn, 45)

- Supporting new/removed attribs in Strava API (hozn, 41, 42)
- Added support for joining/leaving clubs (hozn, 43)
- Respect time zones in datetime objects being converted to epochs. (hozn, 44)