Some more tests

This PR could really use some more eyes, especially for the watch mode test. I'm kind of anxious of all the child_process hackery/tricks as I'm not that proficient in Node.js.

See also #1795.
I really like the way this is all done.  The asynchronous nature of file watching is difficult to capture, but I think you nailed it.

My only concern is that if the test times out for some reason, the `watchProc` won't get killed since that only happens in this listener.  I'm thinking we should move it out into an `after` [hook](http://mochajs.org/#hooks) to prevent any zombies.  Something like:
```javascript
describe('jade --watch', function() {  // enclose the whole suite
  var watchProc;
  var stdout = '';
  it('watch test one', function() {})  // first test
  it('watch test two', function() {})  // second test

  after(function() {  // runs after the suite (both tests) finish
    if (watchProc != null) watchProc.kill('SIGINT');
  });
});
```