Add component "Assumptions" to product "Vision" tab.

I know a new custom Vision component can be added, but "Assumptions" might make a good stock one.  I'd wager a lot of your user's follow the general philosophy outlined in "The Lean Start-Up" and would benefit from having this component smack them in the face.  Before you can test an assumption you have to think about what they are.  I find my self in the right sort of mindset for this when I am completing the Vision tab, so that is where I'm adding it.