Switch license from WTFPL to AGPLv3

[davidbgk]

Rationale: the WTFPL is not an OSI approved license since it has been rejected as such in 2009 (emphase is mine):

Title: WTFPL Submission: http://crynwr.com/cgi-bin/ezmlm-cgi?17:mss:634:200902:aglgcgbhmfcheffmdgon License: http://sam.zoy.org/wtfpl/ Comments: It’s no different from dedication to the public domain. Author has submitted license approval request — author is free to make public domain dedication. Although he agrees with the recommendation, Mr. Michlmayr notes that public domain doesn’t exist in Europe. Recommend: Reject — https://opensource.org/meeting-minutes/minutes20090304/

This is an issue to be able to apply to some open-source programs and grants.