Add VectorOfPostprocessors and test

 ref #4176
OK, I'll admit that I didn't look really hard for examples of doxygen comments before I wrote that previous comment.  After writing that, I found several doxygen comments on methods like these, and a significant percentage of them were flat-out wrong because they were obviously copy/pasted from other classes that did different things.  I totally agree that it's a good idea to put in a doxygen comment saying what the class does, or what some obscure method does, but it seems counter-productive to put them on methods like these whose function is obvious by the name.  From browsing through the code, there is clearly no uniform style to follow on these.