Adds URL to look up JSON-formatted sponsor listing.

Similar to the existing schedule JSON dump, it would be helpful
to have a way to get all the sponsor information programmatically.
This allows for easier updates to e.g. the mobile app when sponsor
info changes.

I'd particularly appreciate a review of the JSON non-ASCII dump
and subsequent utf-8 encoding, since I'm still not super-clear on the
details of Unicode handling in Python < 3.

If nobody objects I'll likely also add a similar endpoint for speaker info.