(WIP) implement sticky_pattern option in unlearner

I implemented unlearning with sticky ID feature, as proposed in https://github.com/jubatus/jubatus/issues/756#issuecomment-42396255 ([design document](https://gist.github.com/kmaehashi/ff8e0498531698f15119)).

WIP: I haven't confirmed that this works on Nearest Neighbor based algorithms (i.e., `recommender` and `anomaly`) with MIX because of blocking issue https://github.com/jubatus/jubatus/issues/806

Note: I'll write document to website later. 
For any ID ``x``, ``x`` will not be in ``entry_map_`` and ``sticky_ids_`` at the same time.
If ``x`` matches the ``sticky_pattern``, it always goes to ``sticky_ids_``. Otherwise it always goes to ``entry_map_``.