Resolves willSelect bug on row selection (issue #502)

[NicolaLC]

According with the issue 502 this pull request resolves the first row selection bug with selectionMode != 'multi'.

This pull request also includes a new Output method for the ng2-smart-table component:

afterGridInit

Called after new Grid method completed, will pass the DataSet as param:

Example

1. Set a callback for the Output:

<ng2-smart-table
        [...]
        (afterGridInit)="onGridInit($event)"
></ng2-smart-table>

2. Do what you need after grid init, for example select a certain row:

  onGridInit(data: DataSet) {
    if ( !this.selectedData ) {
      return;
    }
    
    data.getRows().map(
      row => {
        if ( row.getData() === this.selectedData ) {
          data.selectRow(row);
        }
      }
    );
  }