implement startup loading events (bug 1024014)

The docs are at https://developer.mozilla.org/en-US/Apps/Build/Performance/Firefox_OS_app_responsiveness_guidelines

There some ambiguity in what the events mean especially since we show the splash screen up until everything is loaded. I decided to just trigger/group some of the events together once the splash screen is loaded.
Could add a comment to https://developer.mozilla.org/en-US/Apps/Build/Performance/Firefox_OS_app_responsiveness_guidelines to help anyone who has to maintain this later