timestamps have "not null" contraints by default in rails3.

The records table needs to have the "not null" constraints removed for the three timestamps (created_at, updated_at, change_date) to support SLAVE domains.
pdns has no knowledge of the fields, leaves them undefined when inserting records.

Additionally the schema.rb was updated to to reflect the current state of the migrations.
Merge pull request #47 from databus23/schema_bugfix

timestamps have "not null" contraints by default in rails3.