added start on call and priority for "startables"

When using Startable Facility, I need to start Components in order, that's why I added an option for prioritization of them. Components with lowest priority number are started at first.

I also need to say, when to start startable components, since we apply installers from multiple assemblies. Anyway, I am sorry that these two funcitonalities are in one commit but I made it all pretty fast since it was pretty easy to add it.

If the pull request gets merged, I am also ready to update the documentation. Thanks for opinions and much appreciated code review.