Added test for ampersands escaping

Will leave both double and single quotes unconverted.

Catch by @ErikMinekus

Code:

```php
$code = new Decoda\Decoda('[url]http://milesj.me?a=b&c[/url]');
$code->defaults();
echo $code->parse();
```

Output:

```html
<a href="http://milesj.me?a=b&amp;c">http://milesj.me?a=b&c</a>
```

As you can see the ampersand is correctly encoded within attributes, but not inside the content.
Even though browsers render this correctly, it's not valid HTML.
Fixed ampersands escaping in block contents

Escape special chars without quotes in block contents.

With this patch, PHP will not encode existing html entities.

Maybe will be better to add a `charset` configuration to select the good one.
Merge pull request #77 from alquerci/issue/76

Fixed ampersands escaping in block contents
Added better support for encoding problems when escaping a string (available as of PHP 5.4)

From the PHP CHANGELOG:

The flag ENT_SUBSTITUTE makes invalid multibyte sequences be replaced by
U+FFFD (UTF-8) or &#FFFD; by htmlspecialchars and htmlentities. It is an
alternative to the default behavior, which just returns an empty string and to
ENT_IGNORE, which is a security risk. The behavior follows the recommendations
of Unicode Technical Report #36.