Support for backing up/restoring domain values

[TristanDamron]

With #8, I noticed a bug where domain values were not being backed up/restored correctly. When you back up and restore a table schema that contains a domain, the restore task automatically assumes that the data type for that field is an ordinary whole number, resulting in data loss. Not every data set will have these data types, but it's common enough that we definitely should support it.

To fix this, we need to dump all domain values into the schema file for each table with a domain. The restore task shouldn't need a code change, but I may be mistaken about that.