This introduce a helper class that should be used to log things,
avoiding a lot of boilerplate code.
Also all output are made in /tmp folder instead of beside the script
itself. This is because there is a high chance that the folder containing
the script is not writtable by webserver. So using the /tmp folder
makes it more likely to works in a variety of setup.
All code for unit tests is now under the `PhpSpreadsheet\Tests` namespace
which is autoloaded via composer mechanism. So there is no need for
`require()` anymore.
Also, tests were moved in `tests/` folder and phpunit should be executed from
the project root folder. This is to conform to the de facto standard, notably
in use in phpunit itself.
This is a decision annouced by @MarkBaker in Gitter to be able to
use `DateTimeInterface`.
Also remove deprecated composer option, and introduce caching for
faster CI tests
