Migrate to atoum namespace instead of mageekguy\atoum

Remove all mageekguy reference, expect in classloader to keep retro-compatibility with mageekguy\atoum namespace.

mageekguy.atoum.phar was renammed to atoum.phar

Documentation (readme file) is also modified.

All tests are green.

Is it the biggest PR you've received ?