Add URL to look up JSON-formatted speaker listing.

Similar to the existing schedule and sponsor JSON dumps, it is helpful to
have a way to get all the speaker information programmatically. This allows
for easier updates to e.g. the mobile app when speaker info changes.
Ticket sales close today, and are closed tomorrow (at 6am).
Merge pull request #110 from felixc/speakers-json

Add URL to look up JSON-formatted speaker listing.
Describes the pre-party in vague hand-wavy non-details. Closes #107.