Initial pass at creating BW::UIActivityViewController

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: BW::Constants.get("UIActivityType", :add_to_reading_list)
)

# With completion handler
BW::UIActivityViewController.new(
  items: "Some Text",
  excluded: BW::Constants.get("UIActivityType", :add_to_reading_list)
) do |activity_type, completed|
  puts "completed with activity: #{activity_type} - finished?: #{completed}"
end
```

Let me know if i'm on the right track here.