Code, design, and some graphics by Johannes Häggqvist

"Phantom from Space" by Kevin MacLeod (incompetech.com)
Licensed under Creative Commons: By Attribution 3.0 License
http://creativecommons.org/licenses/by/3.0/

Some graphics by MillionthVector (http://millionthvector.blogspot.com/p/free-sprites.html)
Licensed under Creative Commons Attribution 4.0 International License
http://creativecommons.org/licenses/by/4.0/

Some graphics by webtreats (https://www.flickr.com/photos/webtreatsetc/)
Licensed under Creative Commons Attribution 2.0 Generic License
https://creativecommons.org/licenses/by/2.0/
