Change migrate to use native DataField API

The recommendation for schema migrations are to use the South
`orm` proxy. Data migrations are generally less problematic in
this regard, so the native model is used here to have access
to properties and methods (such as `searchable`).

Fix #227

Signed-off-by: Byron Ruth <b@devel.io>
Merge pull request #228 from cbmi/issue-227

Change migrate to use native DataField API