Support timestamps like a rails

If an entity has attribute created_at and/or updated_at, it is set automatically at saved.
set timestamps in a top context only.
fix a bug. created_at= -> updated_at=
change the default root name. Use CFBundleExecutable instead of CFBundleDisplayName.

CFBundleDisplayName is localizable.
If I added a localized file, the root name is localized.
Then the app will be missing the database file.
Merge pull request #31 from katsuyoshi/db_name

change the default root name. Use CFBundleExecutable instead of CFBundleDisplayName.
Merge pull request #27 from katsuyoshi/timestamp

Support timestamps like a rails