Add currently known information

This contains 3 commits:
1) A quick README tweak to reflect the presence of SVG images.
2) Added DTK, including its release date, added Magic 2016, and exit ("Until") information for KTK, FRF, DTK, and 2016. Also switched exit labels to use Spring/Fall to match Wizards' usage.
3) Bumped API to version 4 and changed API `rough_exit_date` to use Spring/Fall to match Wizards' usage.

Thanks for making this, it's really useful! :ok_hand: