Added support to challenge AI endpoint

[jrwaine]

Added client.challenges.ai function for api/challenge/ai endpoint.

[rhgrant10]

Thanks for this. I added the other challenge methods missing and also gave the create methods some readable and distinguished names:

• create
• create_ai
• create_open
• create_with_accept

All of these are in v0.10.0, now available on the PyPI.