Initial xAPI 2.0 Release

Initial release of the xAPI 2.0 changes.

This update adds support for the contextAgents and contextGroups properties, several updates related to Concurrency for document resources, and other changes mandated by the IEEE 9274.1.1 xAPI Base Standard.

You can read more about xAPI 2.0 here: https://opensource.ieee.org/xapi/xapi-base-standard-documentation

What's Changed

• Updating 2.0 with VDP Patches by @vbhayden in #515
• Adding Last-Modified Headers by @vbhayden in #516
• xAPI 2.0 Update by @vbhayden in #508

Full Changelog: v1.3...v2.0.0