Update example and tutorial notebooks to new version of `petpy`

[aschleg]

The current notebooks in the README are still using the old version of petpy before Petfinder switched to v2.0.
Update notebooks to use new version of petpy and try to align examples as much as possible as the difference between the Petfinder API 2.0 and the legacy version is rather different.

[aschleg]

Introduction notebook has been updated to new version of Petfinder API and petpy.

[zhangluustb]

@aschleg
The v2.0 API says that your current access allows 1,000 requests/day, with a rate limit of 50 requests/second. Is that means it's not possible to use notebook 02 to download 45000 images any more?