Move cake executables into bin directory

Refs https://github.com/cakephp/cakephp/issues/4347

Note: wasn't sure if we should just leave `Installer.php` in `src/Console`... heh, but I did.  Open to suggestions on a better location or directory name for it.
@markstory sure, but the one place would be vendor/bin then, which is achievable by setting `bin` config in composer.json so composer will create a symlink for `app/bin/cake` to `vendor/bin/cake`