Long overdue update to AUTHORS

Adds authors who have contributed to the main branch in the past 3-4
years. The command to get the list of authors, in order of first
contribution, is:
git log --format='%aN' | tac | cat -n | sort -k 2 | uniq -f 1 | sort -n | cut -f 2-
Then I did some manual filtering of already present authors and invalid
names.

Feel free to change the format of your own name if you wish.
Merge pull request #1308 from learning-unlimited/authors

Long overdue update to AUTHORS