Add OSX support

Hello,

I finally got time to extract the OSX port I did.

As I stated before the main difference between iOS and OSX WebView are the delegates.

My goal was to keep the WebViewJavascriptBridge API code in one place while being able to have different delegate method whether the library is used on iOS or OSX.

I've not reworked the documentation too much as it would add lots of redundancy with the current state of the documentation.

Feel free to ask question, give suggestions or even better integrate it into master.

Antoine.