Allow hierarchical app names, like: 'foo.bar'

`./manage.py test` was aways running all tests for me because this project has app names like 'foo.bar'. Turns out `imp.find_module()` does not handle hierarchical modules.

I've created a new method `import_app_module` on nose runner class to handle it.
It could be a function in a separate helper module, my idea is to refactor it later when adding support for other runners, I'd like to add a py.test runner class :)

BTW, nice module Gabriel, always helping to improve testing in python :)