Stylesheet path of HTML coverage report when tested class is in the root namespace

**Given** a class:

```php
<?php

namespace
{
    class foo
    {
        //...
    }
}
```

**And** a test:

```php
<?php

namespace tests\units
{
    use atoum;

    class foo extends atoum
    {
    	//...
    }
}
```

**When** the user enables the HTML coverage report
**And** he runs the previous test
**Then** he gets a broken coverage report:

The directory containing the HTML report looks like this:
```
.
├── foo.html
├── index.html
└── screen.css
```

In the `foo.html` the stylesheet is linked using `<link rel="stylesheet" media="screen" type="text/css" href="/screen.css" title="Screen" />` which is not good: the stylesheet path is absolute from the root (leading `/`) when it should be relative to the `foo.html` file. I fixed this by using a simple `if` checking if the tested class is in the root namespace or not. If it's in the root namespace, leading `/` is not added.