Fixed issues.

- add destroy of class method.

- add transaction to restore of class method.

- alias recover to restore!
refactor test codes.

- split core test.
bugfix dependent association behavior when hard-delete.
bugfix association.

- bugfix association.
- refactor `soft_delete_core.rb`.
bugfix option.

- bugfix option.
- update `README.md`.
- update version.
Merge pull request #3 from alfa-jpn/develop

release 0.1.2