Makes Voikko compile on OS X Mavericks.

The problem was, including both <wchar.h> and <cwchar> and then using
'using namespace std' caused ambiguous versions of wchar functions
to be imported to default scope. For some reason the version of Clang
included with Mavericks is stricter about this.
Merge pull request #6 from osimola/mavericks

Makes Voikko compile on OS X Mavericks.