Missing <cstddef>

On Linux with recent G++ for v0.1.3 I get errors:

```
Source/include/slikenet/HTTPConnection2.h:76:143: error: 'ptrdiff_t' has not been declared
Source/include/slikenet/HTTPConnection2.h:94:17: error: 'ptrdiff_t' does not name a type
...
```

Probably not the best place, but a fix is to add:
    
      #include <cstddef>
    
on top of `Source/include/slikenet/HTTPConnection2.h`