Update migrations.py

Use obj.Schema() instead of obj.schema or some fields (like the ones provided by archetypes.schemaextender) are not taken into account...
Merge pull request #6 from gbastien/master

Use obj.Schema() instead of obj.schema in the migration process
Preparing release 1.5.8
Update changelog.
Back to development: 1.5.9
Merge branch 'original'