Added support for the new "browser" platform in Cordova 4

The new browser platform in Cordova 4 is awesome.   And now this plugin is awesome because it supports it.   :)
I think this will block all of js execution until the dialog is dismissed?
This shouldn't be an issue for many cases, but it will result in a different behavior compared to android implementation where it's completely async. Actually I was working on an async implementation of this for browser. So I would be glad if this goes in and then I'll add my part on top.