Refactoring for hooks to use promises and promise chains

[daffl]

This pull request is a larger refactoring that allows service methods that return promises and turns hooks into promise chains. They can now also be used like this:

app.service('todos').before({
  create(hook) {
    if(!hook.data.text) {
      throw new Error('Text has to be provided');
    }

    hook.data.text = data.text.substring(0, 255);
  }
});

Closes #28

[ekryski]

[marshallswain]

Wow. This really made hooks really clean.