Call register_label when train method does not change state of storage, and return labels whose scores are zero
Make inp method pure virtual
Add labels to stub_storage to maintain all labels
Merge branch 'develop' into return-zero-labels

Conflicts:
	jubatus/core/driver/linear_function_mixer_test.cpp
	jubatus/core/storage/storage_test.cpp
Merge pull request #670 from jubatus/return-zero-labels

Return zero labels