Escape double quotes inside property value

When doing:

```php
echo html_tag('img', array(
    'src' => 'whatever.png',
    'alt' => 'Look at "this" description',
));
// Will output <img src="whatever.png" alt="Look at "this" description" />
```

This is not expected behaviour. My fixed is based on this answer: http://stackoverflow.com/questions/4015345/how-to-properly-escape-quotes-inside-html-attributes

It's still legit to HTML inside attribute values (some JS tooltip plugins work like that), but the quote really has to be escaped.