Add ShellTask for generating shell class stubs.
If names are empty error out.
Add support for generating Shell command test cases.
Fix tests that fail when bake tasks are added.
Remove unused argument.
Remove defunct property.
Remove uneeded namespace registration for class loader.

With composer loader now being used for tests too the extra loader
registrations are not required.
Merge pull request #3360 from ADmad/3.0-exception

3.0 exception
Merge pull request #3361 from ADmad/3.0-tests

Remove uneeded namespace registration for class loader.
Merge pull request #3359 from cakephp/3.0-shell-bake

3.0 shell bake