Fixes #17 - Move String( const wchar_t *, ... ) to static function

Moved String( const wchar_t *, int len = -1 ) to static function to
prevent confusing the compiler when a NULL pointer is used (it could
not differentiate between (const char *)NULL and (const wchar_t *)NULL
)
Add (and pass) unit tests for String::fromWChars
Merge pull request #17 from caitp/issue-17

Make string wchar_t * constructor a static function