Added method to get a column from a table node.

Used by: https://github.com/Behat/Behat/pull/654

Allows you to get a column of a table node by index.

For example:

The Gherkin
```gherkin
          Given I am user:
            | username | ever.zet |
            | age      | 22       |
```

What that looks like in PHP
```php
$table = new TableNode(array(
            array('username', 'ever.zet'),
            array('age', '22'),
       ));
$table->getColumn(0);
//=> array('username', 'age')
```
hmm, indeed. It may be more readable to write it this way though:

```php
if ($index >= count($this->getRow(0))) {
    throw //...
}
```