Changed readme code examples to window.plugins.webintent.*

The readme code examples use "WebIntent.EXTRA_TEXT" as parameters for hasExtra and getExtra. From what I can tell, these parameters should instead be "window.plugins.webintent.EXTRA_TEXT". This is the only format that works for me. I imagine this is how it should now work after .prototype was added to the declarations a few commits ago. So I updated the readme file accordingly.