Add attributes mapped, is_association and data_source

[philippemilink]

Hi, I added some attributes to column, and changed a little bit the way of generating the data response: there is now two rows arrays: the first one contains the data from the database and the second one contains the result displayed in the table. This is useful to reuse the data in another column, to make eventually another transformation on the data. Moreover, we can now use the attribute mapped set to False to create columns based on data from other column.

Requires PRs #816, #837.

[stephanvierkant]

This PR should be merged to fix the broken feature: #839

@philippemilink Can you resolve conflicts* and add the LinkColumn to \Sg\DatatablesBundle\Tests\Datatables\PostDatatable so we know it's working?

Because \Sg\DatatablesBundle\Datatable\Column\ColumnInterface has been changed, we would require a version bump.

*Copy .php_cs.dist and run php-cs-fixer fix before merging, that makes it a lot easier.

[philippemilink]

Since @stephanvierkant made his own PR (#914), can I close this one ?