Webscreen tests

I tried following the [MacBacon instructions](https://github.com/alloy/MacBacon/blob/master/README.md#wait-without-fixed-period-of-time-until-resume-is-called) for testing delegate callbacks to no avail. I kept getting a no method error on my call to `resume`. So i'm just waiting a bit of time to make sure the view was loaded.

In rare instances where you have a really slow internet connection, the test that loads google into the webview may fail.

A few cool thing I added to this pull request:

1. You can now call the `html` method and it will return the html of the webview in the form of a string.
2. There's a new method called `set_content` that you can use to change the content on a whim. I changed the `set_initial_content` to use that method so it retains all the nice `NSURL` stuff.