Can rename and convert column with `repair`

It should be possible to rename columns in the database.
- [ ] add a function `JSONModel::renameColumns`

``` php
$renamed = JSONModel::renameColumns($model, $columns, array(
    'old_name' => 'new_name'
));
```
- [ ] add an optional `$rename` argument to `repairSchemaIter` and `repair`, to rename columns.

Rolling back a name change in a following version is not possible.
