Remove Ohm::Datatypes#bool method.

From Ohm 2, if an attribute is set to false, it's not saved.
Related commit: https://github.com/soveran/ohm/commit/717080f.
Add CHANGELOG.md file.
Fix Ohm::SoftDelete#restore. Remove key instead of set it to nil.
Merge pull request #25 from frodsan/fix_softdelete_plugin

Fix Ohm::SoftDelete#restore. Remove key instead of set it to nil.
Merge pull request #24 from frodsan/add_changelog

Add CHANGELOG.md file.
Merge pull request #23 from frodsan/remove_bool_method

Remove Ohm::Datatypes#bool method.