Explicitly set date added to conform to MySQL 5.6.6

See

https://github.com/omeka/plugin-CsvImport/issues/60#issuecomment-55128347
http://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_explicit_defaults_for_timestamp
Merge pull request #1 from omeka/master

Explicitly set date added to conform to MySQL 5.6.6