SimpleString::StrCmp (2)

1. This implementation works
2. StrCmp and StrNCmp should be similarly implemented (whichever way)
3. More complete tests for both
4. @basvodde Not quite what you had in mind?
How about:

```
    for(; *s1; s1++, s2++)
        if (*s1 != *s2)
            return *(unsigned char *) s1 - *(unsigned char *) s2;
    return *s2 ? 0 - *(unsigned char*) s2 : 0;
```

Passes all tests