Removes static reference to testem.js; Moves testem.js to public

cc @stefanpenner, @pangratz

This is a little embarrassing, but I had a false positive on PR #404, it doesn't actually work. Apparently, the static middleware doesn't quite work as I originally thought. The combination of urlRoot and directory does not work for serving up specific files because [line 93](https://github.com/stefanpenner/ember-app-kit/blob/master/tasks/express-server.js#L93) replaces the file name with ' '.

The solution is quite simple. The express server already looks into the /public folder for any un-handled files. So, just moving the testem.js fake file into /public works, without any modification to the express server. 

I've tested this several times :smile: 

:ok_hand:  grunt test
:ok_hand:  testem
:ok_hand:  grunt server & http://localhost:8000/tests