Allow insertion of empty row using default values

I believe this change speaks for itself ?
Anyway it came from googling
https://dev.mysql.com/doc/refman/5.0/en/data-type-defaults.html
http://blog.jooq.org/2014/01/08/lesser-known-sql-features-default-values/
http://msdn.microsoft.com/en-us/library/ms174335.aspx

it's useful for another PR on node-orm2