Add a grunt watch task

`npm test` runs 24s, makes a lot of network I/O, compiles C modules etc… So it’s not usable for test-driven development. Also, the output is to noise so I can’t see at first glance if the tests passed.

I’d like to have a watch task that just does what the two `coffee --watch` commands did once so I can run the test suite in the browser. This allows me to chose different browsers and select specific specs (`?grep=^SomeSpec`). (If there’s a better way, please let me know.)

I tried to add a watch task, but I’m getting this error:

```
$ grunt watch
Running "watch" task
Waiting...OK
>> File "src/chaplin/views/collection_view.coffee" changed.
Running "coffee:compile coffee:test copy:amd" (coffee) task
Verifying property coffee.compile coffee exists in config...ERROR
>> Unable to process task.
Warning: Required config property "coffee.compile coffee" missing. Use --force to continue.

Aborted due to warnings.
```

Any idea?