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')
```