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

r=etienne