Update the ci_*.sh scripts and move them to the more conventional
ci/ directory. Update .appveyor.yml and .travis.yml, as well as
the AUTHORS file, accordingly.
Speed up ci_cmake.sh: add "-DPNG_TESTS=OFF" to the list of CMake
variables if CI_NO_TEST is true.
Remove "sudo: false" from .travis.yml.
Refactor .appveyor.yml.
