Variables should be able to start with underscore (_)

Don't have time to fork and push right now, but before I forget...

line 173 of /h2o/parser.php:

From:

``` php
        self::$name         = '/[a-zA-Z][a-zA-Z0-9-_]*(?:\.[a-zA-Z_0-9][a-zA-Z0-9_-]*)*/';
```

To:

``` php
        self::$name         = '/[a-zA-Z_][a-zA-Z0-9-_]*(?:\.[a-zA-Z_0-9][a-zA-Z0-9_-]*)*/';
```
