Optimize $indexFor by caching index locations in a weak map
src/FirebaseArray.js: added a cache of record indices
test/unit/firebase.spec.js: Fix broken test case
test/mocks/mock.utils.js: unused parameter
Merge branch 'master' into kato-indexfor

Conflicts:
	src/FirebaseArray.js
Merge pull request #461 from firebase/kato-indexfor

Optimize $indexFor by caching index locations in a weak map