rewrite RMXFirebaseCollection, no longer mirrors Firebase events,
has a higher level interface only dealing with models
add RMXFirebaseModel#changed,
dont wrap once/once_finished in gcd block,
remove RMXFirebaseModel.create adhoc model creation, must subclass,
dont return cancelled models from the identity map
make RMXFirebaseTableHandlerViewCell work as a sizerCell with RMXTableHandler
make default for #pending do reset
simplify ready/cancelled logic, only have :finished trigger
and let user or views decide how to handle cancelled
remove RMXFirebaseDataSnapshot, patch FDataSnapshot directly
to avoid wrapping everything in our own class. make some blocks
that get carried through safe_blocks so they dont leak.
use range for paging, dont do starting/ending comparisons