Fix nested namespaces in 'using' declarations

The problem
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Nested namespaces aren't highlighted in 'using' declarations, as you can see on line 12:

![before](http://files.1337upload.net/Screenshot_from_2014-05-11_22_20_18-c05cd3.png)

The fix
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Add a special rule for 'using namespace' declarations where nested namespaces are allowed. This way nested namespaces are still ignored in places where they aren't allowed.

Line 12 is now displayed properly, and line 5 is still displayed as it should be:

![after](http://files.1337upload.net/Screenshot_from_2014-05-11_22_19_32-e00811.png)