Support ParallelForEach api for scheduling workflow items

[gurmitteotia]

Examples:

public class TranscodeWorkflow : Workflow
{
  public TranscodeWorkflow()
  {
     ScheduleActivity<DownloadActivity>();
     ParallelForEach(_=>new {"MPEG", "WAV"}).ScheduleActivity<TranscodeActivity>()
              .WithInput(i=>new {Path =p.ParentResult().Path, Format= i.Key})
              .AfterActivity<DownloadActivity>()
  }
}

public class ProcessImagesWorkflow : Workflow
{
  public TranscodeWorkflow()
  {
     ScheduleActivity<FilterTheImages>();
     ParallelForEach(a=>a.ParentResult().Images, limit=10).ScheduleActivity<SmoothImages>()
              .WithInput(i=>new {Path =i.Key})
              .AfterActivity<DownloadActivity>()
  }
}