Support save/load of unlearner

[rimms]

Fixes https://github.com/jubatus/jubatus_core/issues/389

I implemented pack and unpack interface to unlearner.

And, I implemented this method call to algorithms:

• [x] anomaly::light_lof
• [x] anomaly::lof
• [x] classifier::linear_classifier
• [x] classifier::nearest_neighbor_classifier
• [x] nearest_neighbor
• [x] recommender::euclid_lsh
• [x] recommender::inverted_index
• [x] recommender::inverted_index_euclid
• [x] recommender::lsh
• [x] recommender::minhash
• [x] recommender::nearest_neighbor_recommender
• [x] regression::nearest_neighbor_regression

I tested this patch using this .

Note that nearest_neighbor does not support unlearner. But driver::nearest_neighbor has the interface using unlearner. I think this method will not be called from anywhere.

[rimms]

Ready for review.

[TkrUdagawa]

LGTM