Allow all-Orchestration on Promises

Hi, we have extended you library with ability to combine the promises with a all-Method, like in JS-Frameworks (e.g. AngularJS).

Examples:
```java
Promise p1 = ....
Promise p2 = ....
Promise.all(p1, p2).then(....)
```
or with timeout:
```java
Promise p1 = ....
Promise p2 = ....
Promise.all(1000 /*timeout in ms*/, p1, p2).then(....)
```

It would be great if you accept these changes and publish a new version to maven central
It should be independent from vertx. Only one dependency should be exists -> RxJava. Btw, I think, ability to start a own threads (even over standard Java Concurrent API, like Executor or Fork/Join) doesn't break vertx. Otherwise it would not be possible (or wrong!) to use fork/join or parallel stream in a worker verticle.

the same applies to third party libraries, that are used in worker verticles. We can not prohibit the use of Executor Service in there.