Use testem as module

In node 0.10 the `stdout` buffer doesn't seem to be drained properly. This makes some tests hang when spawning new processes.
Since testem 0.4.8 it's possible to require and start it providing a custom reporter without the need of spawning processes and capturing `stdout`.

Moreover as of `0.5.0` it's now necessary to add a `#testem` hash to `test_page`.

I've also added a test case to verify that exit code and messages are correct.