bug 939809

* added StackManager.snapshot which gets a copy of the array of applications in the stack
* updated CardsView to get the list of and current app from StackManager
* made StackManager ignore the System application explicitly
* minor changes to help with jshint later
* added 'position' getter on stack manager
* added several tests to ensure that the position is current after stack has changed
* moved apps used in cards view test into a variable used to populate running apps as well as the stack
* removed old user ordering code (web os model task manager...) since it's incompatible with our new model
  and the code does not have any tests and it hasn't been enabled in a long time. We're moving on!

r=etienne