Updated options passed to Collection#fetch()

add: true didn't do anything, updated to update: true, which causes Collection#update() to run instead of Collection#reset()
remove: false added to prevent Collection#update() to not remove existing models. This is important if some models were grabbed from cache and other models relied on fetchRelated() calling fetch(). If remove != false, then the cached models would be removed
Merge pull request #271 from jamesshannon/patch-2

Updated options fetchRelated() passed to Collection#fetch().

Replace `add:true` by `update:true, remove:false`.