toString handles wchar_t raw strings

I use wstring a lot for some projects and I noticed that a lot of times when my tests failed it wasn't printing the string value properly.  For example:

```c++
TEST_CASE( "toString wchar_t fails" )
{
	std::wstring x = L"wide load";
	CHECK( x == L"wider load" );
}
```
Resulted in 
```c++
  CHECK( x == L"wider load" )
with expansion:
  "wide load" == 005A5E6C
```

This pull request adds two toString implementations for wchar_t (const and const pointer const).