docs: update links to point to new karma repo

This updates links in docs (including changelog, contributing, readme, comments, manifest)

 * https://github.com/testacular/testacular/ to https://github.com/karma-runner/karma/
 * http://testacular.github.com/ to http://karma-runner.github.com/
 * https://github.com/vojtajina/testacular/ to https://github.com/karma-runner/karma/