Add VectorOfPostprocessors and test

 ref #4176
I would like to see all classes, method, and member variables have doxygen style comments, regardless of the simplicity of the comments. It is my opinion is that any new code should be completely documented, for the sake of completeness alone. Also, when running doxygen warnings are given for code without comments. Eventually, as we make changes to MOOSE, the non-documented or incorrectly documented code will get updated if we all try to be more conscience of this issue. 

Also, future changes to MOOSE that involve copy and paste should go through a PR, I hope we will catch such documentation problems.

Documentation is my soapbox, we are lacking good documentation and it limits are growth, I hope to change that, but I need help. That being said, if you are that strong against adding the comments I will just add them later. I could of added the lines in less time that it took me to write this comment.