Added support for progress notifications

Progress Notifications are part of the available notifications templates. See https://developer.chrome.com/apps/notifications#type-TemplateType

TEST=
* `cd /tmp`
* `git clone https://github.com/GoogleChrome/chrome-app-samples`
* `cca create myApp --copy-from=/tmp/chrome-app-samples/rich-notifications/`
* `cd /myApp`
* `cca run android`
* Click on "Progress Notification" button
* Observe a progress notification in the status bar

![screenshot_2014-07-09-16-32-40](https://cloud.githubusercontent.com/assets/634478/3525762/14474f92-0776-11e4-8ee8-e0c1da0795ec.png)