Bug 837673 - [contacts] "ready to use" perf measurement r=jlal

- rename ContactsRenderingPerformance to PerformanceHelperAtom
- store all received events in the PerformanceHelperAtom, in a local variable
  instead of a global variable.
- Start refactoring performance helper to make it a instanciable object. In the
  future, it will probably handle the loops too.
- don't send perf events if we don't listen
(cherry picked from commit dac9b2a8a36305844651aba96a3a1904e74e648d)

Conflicts:
	apps/communications/contacts/index.html
	apps/communications/contacts/js/contacts_list.js
	apps/communications/contacts/test/performance/rendering_test.js