Issue 89: warning on using namespace in header

[WimLeflere]

Added a warning when 'using namespace' is used in a header file. The rest of the file is ignored because the output could be incorrect.

[myint]

@r-e-d, probably has a better idea than me on whether this is a good approach.

[r-e-d]

This new warning is fine for me. But I'm not sure that we really want to abort parsing the file.

One solution would be to mark the file as used if it contains any using statement to prevent incorrect header unused warning.