Variable sortable Paramater

This PR allows the `sortable` parameter to be a variable. It uses the same 'concept' for the variable `title` parameter to make `sortable` also variable.

This is needed for cases when you have a very flexible table whose column values are changeable.

```
// Code
$scope.variableColumn = 'name';

// HTML
    <table ng-table="tableParams" class="table">
        <tr ng-repeat="user in users">
            <td data-title="variableColumn">
                {{user[variableColumn]}}
            </td>
            <td sortable="'age'">
                {{user.age}}
            </td>
        </tr>
    </table>
```