Fix listeners with modifiers

[BlooJeans]

Fix listeners with modifiers reusing handles with the same path but no modifiers

We now keep track of the listeners by path as well as modifierString, and to a lesser extent, by eventType

Previously, if we have:
a) db.ref('/messages').on('child_added')
b) db.ref('/messages').limitToFirst(1).on('child_added')
c) db.ref('/messages').limitToLast(1).on('child_added')

In short:
If we were to push a new message onto /messages, the 'b' query above with limitToFirst would fire its child_added listenerwould get called, despite only the last element having changed

Longer explanation:
it will reuse the same javascript subscription, and it will create 3 unique path+modifier references in the native code, to which we'll attach:
ios: only the appropriate listener
android: the multi-listener for all eventTypes

these listeners will fire correctly whenever (compare with a, b & c above):
a) anything gets added
b) when the last item changes
c) when the first item changes
but when any of these three listeners emit an event, all 3 will receive it

[auser]

Is this ready to go?

[BlooJeans]

Not quite- I'll make a new PR soon which will include the immutable refs/queries as well as these commits.

[BlooJeans]

#132