Provide specific attribute for signal event

[gurmitteotia]

To handle "resume" signal a user should be able to write handler like below

public class OrderWorkflow :Workflow
{
   [Signal]
   public WorkflowAction Resume(ResumeDetail details)
   {
     // take some action and return workflow action.
   }
}

Or

public class OrderWorkflow :Workflow
{
   [Signal(Name="resume")]
   public WorkflowAction OnSignal(ResumeDetail details)
   {
     // take some action and return workflow action.
   }
}

• Matching of signal name should be case insenstive
• Fallback to [WorkflowEvent(EventName.Signal)] handler when no specific SignalAttribute matches.