Column definitions should be non-nullable for native non pointer types.

When creating new tables the columns for non-pointer types are nullable in the database (at least for MariaDB). In the case that inserts/updates only goes through the API this is can be negated in terms a data integrity issue - but this cannot be guaranteed and it will certainly cause problems with the API to assign nulls to non-nilable fields.

From the performance end it may be implementation specific but mysql is faster with non-nullable columns (http://stackoverflow.com/questions/471367/when-to-use-null-in-mysql-tables).

This is addressed in the createTable of https://github.com/jmoiron/modl/blob/master/dbmap.go#L180 and is dependant on the ColumnMap here https://github.com/jmoiron/modl/blob/master/tablemap.go#L295 .