Native support for conditional iteration

Ported from: https://github.com/GoogleCloudPlatform/DataflowJavaSDK/issues/50

There are a variety of use cases which would benefit from native support for conditional iteration.

For instance, http://stackoverflow.com/questions/31654421/conditional-iterations-in-google-cloud-dataflow/31659923?noredirect=1#comment51264604_31659923 asks about being able to write a loop like the following:

PCollection data  = ...
while(needsMoreWork(data)) {
  data = doAStep(data)
}

If there are specific use cases please let us know the details. In the future we will use this issue to post progress updates.