OperatorWeakBinding to not use WeakReferences anymore

related issues:
https://github.com/Netflix/RxJava/pull/1008
https://github.com/Netflix/RxJava/issues/1006
https://github.com/Netflix/RxJava/issues/979
OperatorWeakBinding -> OperatorConditionalBinding
Move bindFragment call in sample to onCreate

Having it in onViewCreated is fine, but misleading, since we don't need to reattach retained fragments. (the referenced object remains the same.)
Merge pull request #1021 from mttkay/fix-operator-weak-binding

OperatorWeakBinding to not use WeakReferences anymore