added `AppWindow.fullscreen()` + test case
add optional `state` to `create` + test case

TODO: we need to dartdoc the `create` method
test cases for `create` in `state` 'minimized' and 'maximized'
dartdoc the `create` function
mark version `0.0.2-dev` and start entry in CHANGELOG
implement `AppWindow.contentWindow` with an `HtmlWindow` class + test cases.
CHANGELOG : fixing version number for next version
Merge pull request #47 from rmsmith/app_window

more work and tests for AppWindow