Implements fixes and allows importing in a relation manager

Merge pull request #30 from eighty9nine/eighty9nine-patch-1

Fix BelongsToTable import bug

Adds table compatibility

- Fixes mutation before validation

- adds option to retain before validation mutation

Adds ability to push data to custom import

Merge pull request #24 from natino45/patch-1

Add afterValidationMutator property

Adds mutate before and after validation

Merge pull request #22 from danimardiana/3.x

fix setCollectionMethod on null

WIP

Merge pull request #19 from kirkita/3.x

export visibility property of file upload