Adds BW::UIActivityViewController for a more Ruby-like interface to activity view controllers

This is my first attempt. Some guidance on how the DSL should work would be greatly appreciated. I'll write tests once I've nailed down functionality.

Usage:

```ruby
# Without a completion handler
BW::UIActivityViewController.new(
  items: "Some Text", # or ["Some Text", NSURL.URLWithString('http://www.rubymotion.com')] or a UIImage
  excluded: :add_to_reading_list
)

# With completion handler
BW::UIActivityViewController.new(
  items: "Some Text",
  excluded: [:add_to_reading_list, :print, :air_drop]
) do |activity_type, completed|
  puts "completed with activity: #{activity_type} - finished?: #{completed}"
end
```

Let me know if i'm on the right track here.