enhancement of Version 1.0 web screen

Hi, I want to be merged this, please.

When I have created RubyMotion app (that is called [shiori](https://github.com/ainame/Shiori)) by ProMotion v0.7.6, I  created [my original webscreen module](https://github.com/ainame/Shiori/blob/master/app/screens/web_view_screen_module.rb).

I think that PM::WebScreen is more better than it.
But, my original webscreen module has some convenience methods, such as open_url, curreent_url, on_rpc_call, etc...
So, I porting some methods from my module to ProMotion::WebScreen.

I implement following methods.
* open_url(url_string) 
  * request to url in WebView
* current_url
  * can get current web site url
* on_request(request, type) hook
  * this hook method is convenience, when you create web bridge.
  * sample code https://github.com/ainame/Shiori/blob/update_promotion/app/screens/viewer_screen.rb#L50