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.
I bet it will move phpunit as well. That's not necessarily a problem, but something we'll have to update in the documentation.