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]

It seems like it would be better if self._find_using_namespaces() returned True if it finds a problem. That way you could do something like:

if not self._find_using_namespaces():
    included_files, forward_declarations = self._read_and_parse_includes()
    self._find_unused_warnings(included_files, forward_declarations)

The above doesn't rely on the self.warnings, which might be modified by other methods (at least in future implementations).

[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.