add rmext_context that yields the context but doesnt retain itself
added rmext_weak_attr_accessor for accessors that hold a weak reference,
added rmext_observe which uses bubblewrap's kvo but places the observation
  on a dummy proxy class instead, so it can watch for the intended observer
  to dealloc and clean up after itself, avoiding a retain cycle
added rmext_performBlockOnDealloc which can take a block to be executed
  before an object is dealloc'd
rmext_performBlockOnDealloc expects to be given a block instead of
passed a block as an argument
add bubble-wrap as a dependency (for kvo), bump version