Add support for using direct functions on column definitions

Row(model)-wise support for editable, renderable & sortable.
Analogous with sortValue.
example:
{"name":"amount_order",
 "editable":function(model) {
    return model.get('status') < 2
 },
 "cell":"integer",
 "label":"Tilaus"
}
Evaluate BooleanCell's checked to native boolean value

jQuery seems to evaluate 0 and "" to checked="checked"
Revert "Evaluate BooleanCell's checked to native boolean value"

This reverts commit baaf081973165853dabd7fae40028f01d9b94dfe.
Merge pull request #387 from pekkast/master

Add support for using direct functions on column definitions for editable, renderable & sortable