Allow users to specify their own nib and bundle names.

This is preliminary support for the [`ib`](https://github.com/yury/ib) gem.  I have [an outstanding pr](https://github.com/yury/ib/pull/45) that fixes their side of generating the xcode stub files for promotion.

You can create an XIB file and initialize it like so:

```ruby
my_cool_screen = CoolScreen.new(nav_bar: true, nib_name:"CoolScreen")
```

I'm wondering if there's a smarter way to do this like search the bundle path for an interface file named `PMScreenSubclassName.xib` and auto-load it if it exists.