Add Time Trigger API and client (#153) (#161)

This change adds a timer trigger API, client, and implementation.  Users can trigger a function with a cron-compatible string. 

This change also moves the publisher interface and webhook-based publisher implementation out of kubewatcher and into a separate `publisher` package.

resource.go

@@ -28,6 +28,10 @@ func (ht HTTPTrigger) Key() string {
 	return ht.Metadata.Name
 }
 
+func (tt TimeTrigger) Key() string {
+	return tt.Metadata.Name
+}
+
 func (w Watch) Key() string {
 	return w.Metadata.Name
 }