Update license to GPLv3; add credits

- Change license to GPLv3 to match the rest of the Censorship Monitoring Project
- Add credits for the third-party components we've reused

Discussion of licensing is at openrightsgroup/cmp-issues#15

To do:

- Confirm we've credited everything we've used (RabbitMQ?!)
- Change declarations in individual file headers if there are any.

/cc @dantheta, @NetworksAreMadeOfString