Detail to fix angular generated by yeoman

Hi there and thanks for sharing grunt-express and grunt-express-example!
I used them to add to server angular as generated by yeoman.

One thing that got in the way is the configuration of the static folders.
It turns out that making the directories browsable prevents the twitter-bootstrap-saas served from the '.tmp' to work as expected.

Here is the project where I am testing this:
https://github.com/hmalphettes/yeoman-angular-express-example

Let me know if I missed something trivial.