Add vert.x compatibility.

This pull requests adds compatibility with [vert.x](http://vertx.io/). Vert.x uses its own timers instead of setTimeout but fortunately only a small addition is needed to make rsvp.js compatible with vert.x. I tried to orient my changes on the way rsvp checks for the availability of web workers but I think it is necessary to put the vertx require inside a try/catch because there is no vertx package but vert.x comes with its own module system.

I also needed to tweak the browserify options ins package.json so that browserify stops complaining about requiring vert.x in rsvp.js-source and add an empty vertx-file.

Hope this is useful. I would be happy to add meaningful tests for this but I am not sure how to test this properly because this only checks if it is possible to require vert.x and then uses the vert.x's timer instead of setTimeout.
promises should run on the micro-task queue.

```js
var vertx = require('vertx');
var vertexNext = vertex.runOnLoop || vertx.runOnContext;

function useVertxTimer() {
  return function() {
    vertexNext(flush);
    };
}
```