Initial Webscreen Support

Based on #150.

I like using html pages to make about pages because of the auto-layout and the ease of updating the content.

Here's some sample usage:

```ruby
# Whatever
open AboutScreen.new(nav_bar: true, external_links: true)
# More whatever

class AboutScreen < PM::WebScreen
  title "About"
  def content
    # This can be a URL String, an NSURL, a string, 
    # or a file name that references a file in the resources directory
    "AboutView.html"
  end
end
```

There's a nifty feature for retina devices that if you are using a file from the resources directory **and** there are images in the html, it'll search for `imagename@2x.ext` and automatically replace it inline if the file exists.