merged branch char101/fix-debug-lineno (PR #894)

This PR was squashed before being merged into the master branch (closes #894).

Commits
-------

7c5854b Fix twig error lineno off by 2

Discussion
----------

Fix twig error lineno off by 2

It seems to me that the lineno reported by Twig_Error is off by 2 from the real lineno in the generated PHP source

For example:

```
{% if true %}
Yes
{% endif %}
```

```php
<?php

/*  */
class __TwigTemplate_d41d8cd98f00b204e9800998ecf8427e extends Twig_Template
{
    public function __construct(Twig_Environment $env)
    {
        parent::__construct($env);

        $this->parent = false;

        $this->blocks = array(
        );
    }

    protected function doDisplay(array $context, array $blocks = array())
    {
        // line 1
        if (true) { // <- this is line 19
            // line 2
            echo "Yes // <- this is line 21
";
        }
    }

    public function getTemplateName()
    {
        return null;
    }

    public function isTraitable()
    {
        return false;
    }

    public function getDebugInfo()
    {
        return array (  19 => 2,  17 => 1,); // <- should be return array (  21 => 2,  19 => 1,);
    }
}
```

---------------------------------------------------------------------------

by char101 at 2012-11-08T08:23:57Z

Since `$compiler->addDebugInfo()` is called first before writing the node content, usually the number of lines in the PHP code is less than 2 (1 for the // lineno comment, and 1 for the generated content) than the line where the node content is written.