Revert "MarkerCollection: Do not add child markers to map"

This reverts commit bfa18f20306e54bcc8fe74891ca54fc0c9a1fe83.

- If child markers aren't added to the map,
  MarkerCollection will only work when using a
  ClusterStrategy

- The reverted commit was attempting to improve performance,
  but I believe the actual performance culprit in that case
  was the Google Dev Tools "async" call stack feature.
MarkerCollection: toggle clustering behavior
Pull out integration tests helper (getMarkers)

- so we can use it for other tests, to
Prevent aeris.Collection from passing unwanted options to models

Consider:

  new aeris.Collection([{ model: 'A' }], { strategy: null })

our intent here is to create a collection with no strategy.
However, all options to the Collection get passed on to any models
created by the Collection constructor. So in this case, our model
would be created with a null strategy. This commit fixes that, so only
the standard collection options are passed through.
Increase limit on SubsetCollection performance test
MapObjectCollection: set map to null on init

- previously, map was undefined on init
remove unused code from spec
gmaps spec: do not override global google object

- we were stubbing google.maps.event outside of a
  test suite scope, which caused it to be applied globally.
Grunt: ignore changes to closure-linter-wrapper after running gjslint