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.