ci: Add a new top-level directory, dedicated for CI verification

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 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.
6 files changed