Add better loading/loaded tracking

Instead of using a tally of items that are loading, store a hash of
id:boolean pairs, each pair representing the loading state of one item.
Fix indicator removal loop

Change to use more efficient loop structure.
Remove event related nonsense

I understand Leaflet Events better now. This removes some silliness I
added due to my lack of knowledge.
Tabs -> spaces
More compatible check for members of _dataLoaders

Previous method was not IE 7/8 compatible.
Remove redundant comments