Improve QString integration in custom_string.cpp

Improve QString integration in custom_string.cpp
There is an example of how to use Qi with Qt's QString as a custom string.
This is very helpful but it doesn't work in conjunction with the debug facilities.
The following patch fixes this by adding a couple of template specialisations.

Patch originally submitted as https://svn.boost.org/trac/boost/ticket/8846
Merge pull request #75 from mloskot/develop

Improve QString integration in custom_string.cpp