Ignore cell property

This feature allows the user to add an `ignore-cell` attribute to a cell to have ng-table not include it in it's column calculation. 

My reason for wanting this functionality is I'm attempting to add expanding rows when one is clicked on by this markup:

```html
<table ng-table="tableParams">
    <tbody ng-repeat="one in many">
        <tr>
            <td title="ng">{{one.first}}</td>
            <td title="table">{{one.second}}</td>
            <td title="is">{{one.third}}</td>
            <td title="awesome">{{one.fourth}}</td>
        </tr>

        <tr>
            <td colspan="4" ignore-cell="true">
                <div><h3>{{one.header}}</h3></div>
            </td>
        </tr>
    </tbody>
</table>
```

This PR will prevent ng-table from creating a table header via the colspan row/cell.