Move lib directory.

Make the old lib directory the new repository root. This makes
installing with composer simple and reduces the directory depth in the
project.

Remove the App/ directory as it has been split into cakephp/app.
Fix paths in Test/init.php

There is one fewer directory now, so update the paths.
Fix failing tests caused by directory changes.

A few tests relied on the lib/ directory, update them.
Fix i18n tests.

The i18n class needs to be initialized with an empty language value or
it will inherit the first used language causing all the tests to fail.

Force start the session to ensure the tests depending on sessions work.
Merge pull request #1633 from markstory/3.0-dir-layout

3.0 Remove app and change directory layout