Implementation of "Exact Matching" on column-based searches

My first pull-request, so pardon if I miss something here.  This one is a fairly self-explanatory addition.  Basically, you can use a new `setExactMatchColumns()` method to specify which column names should use exact matching rather than a partial (LIKE) match.  This arose out of need, as I wanted most of my columns to be searchable via LIKE expressions, but others to be via exact matching.

Pseudo-example:
```
    return Datatable::query($stuff)
        ->showColumns('id', 'field1', 'field2', 'field3')
        ->searchColumns(array('id', 'field1', 'field2', 'field3')
        ->orderColumns('id', ''field1', 'field2', 'field3')
        ->setExactMatchColumns(array('field3'))
        ->make();
```