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.
@lorenzo They would still all in same bin folder. Just `vendor/bin` instead of `app/bin`. As @ceeram suggested we could symlink `app/bin/cake` to `vendor/bin/cake` using post install script.