Add VectorOfPostprocessors and test

 ref #4176
I'm not sure there's too much to say about these.  I'm looking through the MOOSE code base, and haven't yet found any Doxygen comment on an initialize, execute, or finalize method.  I don't think it's helpful to say why they're empty here because that's going to be a lie as soon as someone adds something in the .C file and neglects to remove my comment.  Please point me to an example of a .h file in MOOSE with Doxygen comments about methods like this that I can use as an example, and I'll follow that example.