Rework presentation method to avoid improper usage of view controllers

[nickoneill]

Most of the discussion about what's wrong is in #127. Short version:

Here's what I was seeing when I set viewControllerForAlerts: the pscope UI showed up OK, but then clicking on a permission showed the native permissions alert behind the pscope window.

Normally these native alerts are presented on top of the pscope view which works, despite giving a "don't present view controllers on deteached view controllers" warning (because I added the pscope view manually to the current view controller, not through the proper presentation).

I think the correct thing to do here is modify pscope so that it's a view controller presented modally with that setting (the name is on the tip of my tongue...) that allows us to see to the view controller underneath, so it looks like the current implementation. We can customize the presentation with the animated transitioning API if necessary.